Why are some CS:GO weapons considered "noob guns"?

Noob guns are usually called that way, because they're hardly used in e-sports matches and getting killed by it makes people look bad (which actually isn't true at all, they're just mad). The Counter-Strike community can be very toxic when it comes to weapon choice (equal to the MP7 in previous CoD titles), because they're pretty easy to use in public games. I've seen people calling out everyone who did't use M4 or AK, because they're the weapons everyone "has to use".

The weapons you mentioned are really easy to use, but they don't work everywhere:

Auto Snipers require a decent spot, some coffee and just enough time to wait for victims.. someone will eventually run into you and is most likely dead soon. So a camper has the biggest profit = "noob", because they just sit there camping all the time. You can easily defend a bomb site with it, even against a rush. Doing that with a normal sniper is way harder.

P90 is a typical run and gun weapon and pretty cheap to buy as well. Personally I like it a lot on fast paced maps or to counter eco rounds. Throw a flash into a tunnel or room, jump in and spray & pray - ace. It's not a noob gun at all, but people think it is, because you can do a lot of damage with less aim than with rifles, but those just work differently.

The shotguns are indeed unwanted in a lot of games, because you need no aim at all to kill people. What they forget is how to counter them. Just don't go near them, because they're only effective in close proximity. Defending a bomb site with it can be fun, but the attackers will hate you for it, so it's a "noob gun" because you chose the right weapon for that round.

Also there is no list of what's a noob gun and what's not, because this is highly subjective. It's more a selection of what weapons work in what situation and that always depends on map, enemy and economy. Just don't buy Heavy MGs too much. The impact on your eco is too big to be useful.

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.

Although the submitted answer by dly is quite on point...

The problem with the "noob weapons" comes from older CS games where the (Terrorists in particular) auto-sniper was very frowned upon as it was very easy to hit shots with, had incredible damage output and was generally less-skill per kill. Also the no-scope accuracy while jumping ruining crouching etc was I'd say about 100%.

This has then evolved in (as stated in the other answer) considering pretty much any weapon other than AK/MK/AWP as noob weapon.

People don't like investing into a full buy, MK, nades, 100armor, maybe even a pistol and a zeus just to die by a random spray and pray from the enemy on the other side of the map. When this would happen people would very frequently get very mad and try to vent anyway possible, main one calling out the enemy for their low skill and noob weapon.

Those same people don't realise that the spray-and-pray noob weapons are less accurate, have less damage and would loose out against a good AK/MK player 99.9% of the time.

Those same people also don't realise that maybe while they were practising their one taps against players with AKs and M4s, I was practising my run and gun control against the same AKs and M4s. Maybe, just maybe I deserved to kill them as I've put the time to not only control the spray pattern, but to also control it while both you and me are moving. Maybe I deserved to kill you with a much less superior weapon because you failed your initial engagement with me and didn't stop me with a well placed headshot.

But people don't go into the why, that deep. They are just salty that they died from seemingly less skilled player.

Play with any weapon you like and don't concern yourself with the saltiness of other players. When they die like that, they get mad. When they are mad, they make even more mistakes, and more mistakes means more wins for you.