Why are some CS:GO weapons considered “noob guns”

I can't speak too much past what others have said on what specifically are seen as "noob guns" in CS:GO, but one thing I haven't seen mentioned yet that I think factors into what eventually get called as such are their skill floors and ceilings. In other words, how effective is a weapon when you first pick it up versus after you've used it for tens, hundreds, or even thousands of hours?

A classic example is the "Noob Tube", i.e. the under-barrel grenade launcher attachment, of old COD games (I've only played MW2 and Black Ops, it may or may not still be a thing). The skill floor was practically zero because you could just aim in the general vicinity of someone, out to decent ranges, and often get a one-shot kill. On the other hand, if I remember correctly, it was slow to reload and had a small ammo capacity, so past that first potential kill, you weren't likely to get a whole lot more out of it. Plus, while it was some degree of effective out to mid ranges, it was literally a random lob-shot anywhere past that. Thus, it was satisfying for beginners because at least they had a good chance of getting on the board with each spawn, but it was also unlikely to allow you to carry a game. Almost any other gun would provide you with higher potential and flexibility once you got good at the fundamentals of the game.

On the other hand, whatever happens to be the most powerful assault rifle in a given game tends to have high skill ceiling. The AK in CS:GO is a guaranteed one-shot headshot, which means a higher-skilled player using it gets rewarded with shorter kills, leading to more health left over to finish off the next guy. The AWP in particular is a great example, because someone with good reflexes and general ability to click on people can get incredible results from it. But, in order to be good with these weapons, you usually have no choice but to practice with them and understand the subtleties of that specific gun's mechanics. Not always, but much of the time, playing with other guns just won't translate much towards being effective with the theoretically strongest ones.

This of course goes past just guns in FPS's. It extends into nearly every competitive game, where certain weapons, tools, or characters provide more immediate results at the cost of eventually not being able to improve because you've more or less "mastered" them. Another example I'm familiar with is Overwatch, where heroes like Genji and Ana are very mechanical and difficult to learn but have almost limitless potential, while others such as Mercy and D.Va (the latter of which I mained for quite some time) are easier to grasp but can rarely pull off the really insane stuff that the first two are capable of, no matter how good you get with them.

So while certain weapons that get labeled as "noob" guns can be very effective at what they do in certain situations (and often should be picked up at those times), they can also become a crutch if you use them at the expense of getting practice in with the guns that are harder to use but also have a higher potential. Personally, I've never really been great enough at a given game to make skill ceilings matter, but I'm sure everyone here knows that many, many people online think they're the next MLG pro and think that everyone must play as such.