The Dungeon Master's Guide section on Hades (page 63) provides a likely answer: Larvae
A gray land with an ashen sky, Hades is the destination of many souls that are unclaimed by the gods of the Upper Planes or the fiendish rulers of the Lower Planes. These souls become larvae and spend eternity in this place that lacks a sun, a moon, stars or seasons.
...
A larva is a miserable fiend that retains the facial features of its previous form but has the body of a fat worm. A larva has only a few faint memories of its previous life and the statistics in the larva stat block.
Hades is crawling with larvae. Night hags, liches and rakshasas harvest them for use in vile rituals. Other fiends like to feed on them.
How do they get there? Easy: The River Styx. From page 58:
This river bubbles with grease, foul flotsam, and the putrid remains of battles along its banks. Any creature other than a fiend that tastes or touches the water is affected by a feeblemind spell...
The Styx churns through the top layers of Acheron, the Nine Hells, Gehenna, Hades, Carceri, the Abyss, and Pandemonium. Tributaries of the Styx snake onto lower layers of these planes.
The river can double as a source of water for fiends, who would probably find its foul nature refreshing.
So it's not hard to imagine that devils might have settlements near the Styx, just like humans tend to build towns and cities along bodies of water, and low-ranking devils could be sent to collect larvae periodically. Or perhaps some other enterprising fiends sell larvae to them, just like some have taken up jobs as ferrymen on the Styx.
There may also be portals leading directly to Hades (or indirectly through Sigil) and devils might build settlements "inland" from the Styx around such portals. Again from page 58:
Traveling between the Outer Planes isn't dissimilar from reaching the Outer Planes in the first place. (...) Most often, though, characters use portals - either a portal that links the two planes directly or a portal leading to Sigil, City of Doors, which holds portals to all the planes.
Starving may not be a concern for most demons. The Monster Manual suggests most don't last long due to their constant infighting. They might even cannibalize the other demons they defeat.
A demon might spawn as a manes, then become a dretch, and eventually transform to a vrock after untold time spent fighting and surviving in the Abyss. Such elevations are rare, however, for most demons are destroyed before they attain significant power. The greatest of those that do survive make up the ranks of the demon lords that threaten to tear the Abyss apart with their endless warring.
The fewer long-lived demons likely just force lower demons to get food for them:
Demons respect power and power alone. A greater demon commands shrieking mobs of lesser demons because it can destroy any lesser demon that dares to refuse its commands.
There should be no shortage of manes to do menial tasks, and given that they're weak and the Abyss constantly spawns them from the souls of damned mortals, they're probably considered expendable.
Finally, don't forget that the descriptions of the planes are written from the perspective of denizens of the Material Plane. The Abyss and the Nine Hells may be inhospitable to humanoids but that doesn't mean there's nothing there that fiends can eat. The DMG mentions some layers of the Abyss that might support demon "life" without having to travel all the way to Hades:
The Gaping Maw. Demogorgon's layer in the Abyss is a vast wilderness of savagery and madness known as the Gaping Maw, where even powerful demons
go insane with fear. Reflecting Demogorgon's dual nature, the Gaping Maw consists of a massive primeval continent covered in dense jungle, surrounded by a seemingly endless expanse of ocean and brine fiats.
Death Dells. Yeenoghu rules a layer of ravines known as the Death Dells. Here, creatures must hunt to survive. Even the plants, which must bathe their roots in blood, snare the unwary. Yeenoghu's servants, helping to sate their master's hunger as he prowls his kingdom seeking prey, capture creatures from the Material Plane for release in the Gnoll Lord's realm.
Given the Abyss's supposedly-infinite layers, there's potential for many such layers to exist.
The spell does not say, so it is up to your DM
You create 45 pounds of food and 30 gallons of water...the food is bland but nourishing...
There is no indication in the spell what the food consists of. As such, you must talk with your DM and decide if it makes sense for the spell to allow for the creation of bland but nourishing meat.
As Jeremy Crawford has said:
A spell's text details the spell's effects—the only thing the spell does. Any additional effects are up to the DM.
In my opinion there is nothing game-breaking about allowing a food-creating spell to create some sort of meat. It is, after all, the purpose of the spell to create things to eat. In other words, I see no reason that would justify restricting player freedom and choice in this case.
"Nourishing" food could even make meat vs not-meat irrelevant
In fact, I think it is even reasonable to read nourishing ("containing substances necessary for growth, health, and good condition") as indicating that it doesn't even matter. If the food is nourishing for anybody that eats it, then even a carnivore would be able to subsist on it no matter what form it takes. (If a carnivore can eat something and find it nourishing isn't it essentially meat?) As a DM, this means that I would see nothing wrong with allowing the player to theme the food/water however they like.
However your DM may disagree. Talk with them and work it out.
Best Answer
Because that was that power of one of its eyestalks in previous editions of the game.
In AD&D book Monster Manual II (1983) by Gary Gygax, the ability to create food and water is one of the spectator's four eyestalks, the others being cause serious wounds, paralyzation ray, and telepathy.
The special quality of "Undead Traits" didn't really exist until D&D 3e (2000), which took design influence from Magic: the Gathering to apply general traits to specific monster types. The fact that undead don't need to eat or sleep wasn't really considered relevant by the designers of the very early editions of the game, when D&D was largely about fighting monsters in dungeons rather than any kind of realism.
In the D&D third edition book Magic of Faerûn (2001), the spectator similarly has the ability to create food and water with one of its eyestalks. Its other three were inflict serious wounds, hold monster, and suggestion. An updated version of the creature in Lords of Madness (2005) changes the creature so it creates food and drink as an innate ability, and one of its eye stalks instead causes fatigue.
It needs to eat and drink because according to its lore, the spectator is dedicated to to guard a specific place or thing without the luxury of leaving to hunt for food. Beholders are traditionally a type of living creature known as an Aberration, and therefore it would not be correct to simply make them Undead. Aberrations frequently have to eat, and the large mouth of beholder-like creatures strongly suggests that it is a creature which must eat to survive, and perhaps even greatly enjoys eating. It's therefore highly thematically appropriate for the spectator to be able to create food.
Giving creatures the ability to cast a single spell is a long-standing D&D mechanical tradition that dates back to the earliest editions of the game, when the easiest way for designers to give a monster a magical capability was simply to allow it to cast a spell normally available to a player character spellcasting class such as the cleric. D&D third edition often did this, where they were generally known as spell-like abilities.
It's not unthinkable for living creatures to be immune to the need to eat and drink, of course. The elan are a race of people who are of Aberration type, and they can sustain themselves without food or drink.