Seems legit. Real people having a hard time finding meals can eat like that, and hard-luck adventurers are the kind of people who might see a lot of time between meals.

For someone who can afford to eat though, going hungry is foolish. Having a full belly when you have the opportunity to get one is just a matter of being prepared for the worst when you're venturing into the unknown. You never know when your mule with all the food is going to get eaten by a dragon or your pack might get dropped down a chasm, and then your carefully-timed starvation diet becomes actual starvation, with who knows how long before your next meal.

Mechanically, it would not be unreasonable for the DM to rule that being hungry can cause Disadvantage under the right (or wrong, I suppose) circumstances. If you haven't eaten for three days and you're sneaking through the orcs' kitchen, even revolting cooking smells are potentially distracting and might make the difference between failure and what would have been success—which is nicely modelled by Disadvantage. Similarly, exhausting activity such as climbing or swimming might be harder when your body is being systematically denied fuel.

So yes, you only need to eat every four days to avoid starvation. But as we well know from real-life experience living, going hungry sucks and is worth avoiding if you can, especially if you have any doubt about where your next meal is coming from.

No, otherwise it would be mentioned by the spells' descriptions.

That being said, I believe the reason why it's not mentioned are the spells' intended usages / their themes.

Lesser Restoration allows you to remove the following conditions:

disease, [...] blinded, deafened, paralyzed, or poisoned

All of them are physical conditions, leading me to believe that the intent of the spell is to be your all-rounder spell for minor illnesses etc. Frightened is not a physical condition, therefore it doesn't fit the theme.

Moving on to Greater Restoration, it can remove the following effects:

• One level of exhaustion
• One effect that charmed or petrified the target
• One curse, including the target’s attunement to a cursed magic item
• Any reduction to one of the target’s ability scores
• One effect reducing the target’s hit point maximum

It removes major detrimental effects, and once again, they are all physical effects (and curses), aside from the Charmed condition. In my opinion, it would be reasonable to include Frightened alongside Charmed, but since Charmed can be way more impactful than being Frightened (in one case, you run away, in the other, you might fight for the opposing team), that's probably why they chose Charmed instead of Frightened.

In addition to the previous paragraphs, there are spells that do help against the Frightened condition (sometimes limited to against specific types), just not the Restoration spells:

• Protection from Evil and Good: immune against being Frightened (and other effects) by aberrations, celestials, elementals, fey, fiends, and undead, and advantage on saving throws for existing effects.
• Aura of Purity: advantage on saves against Frightened (and other effects)
• Heroism: immunity against the Frightened condition
• Heroes Feast: immunity against the Frightened condition
• Calm Emotions: can suppress the Frightened condition for its duration, i.e. up to 1 minute. Effect resumes afterwards, unless the duration has run out.

There are probably a few others that I failed to find.