Why are the people that beg to have a certain role...

I believe it's because ''they only know how to play the said role'' and if they really only know how to play 1 role.. then they are most likely not really good at the game.

Not sure if you're in ranked or in normals.

In nornals: Most likely that they know they suck at the role and are trying to practice it.

In Ranked: No idea. Perhaps they think a certain champion is "OP" and expect to win for free even though they've never played it. Perhaps they've played that role with a certain 'cheese' strategy that has gotten them to their current ranking but now players know how to play against it and will stomp them. Most likely it's a combination of the above, and the fact that if you only want one role you may have okay mechanics on your champion but you have shit for game knowledge. If you only ever play top, how are you going to know when the enemy jungler is going to gank, when mid is going to roam, when bot needs you to TP, etc...?

possibly because if they make a mistake earlier they're more likely for someone to jump on them, causing them or their allies to tilt and make mistakes more often.

possibly because our brains tend to forget things we pay less attention to, and when someone asks for a role we remember if they do poorly but if someone just does poorly we're more likely to forget.

possibly because players who are over confident tend to be players who ask for a role, and while in most cases confidence leads to better play, in some cases when you're faced against an opponent stronger than you think it may lead to punished aggression.

They likely don't know how to play anything else.

Therefore, they are not good at the game.

Therefore, they feed.