I figured out why we get that one or two guys who are completely useless on a champion in your games

Because that guy is you.

Now hear me out. You might play well or decently or terribly on your mains but you'll lose nonetheless, and/or you'll get shit talked, and you might shit talk back, and that all will set you on tilt. Once you're on tilt, having lost all of those games with your mains will demoralize you and remove a desire to play on those champs, so you will try ones you're not good at just because they seem like they would be good in the meta.

I found myself playing Karthus mid yesterday when halfway into the game I realized I was completely useless. Sure my score was 3-3 and I was keeping up in cs, but I couldn't really hit Qs, couldn't really teamfight, and my ulti maybe only killed 2 people because my teammates never got close to killing the enemy. You might only do this once a month or so but all the other thousands of players in this game go on tilt every game and you're bound to get them. Would you rather have the 2nd time Rek'sai who tells you 'gg we lost' after the score goes 0-2 or someone who just takes it and stays quiet? Tilt plays a huge factor in games, way too huge, so if you find yourself desperate or wanting to try new strategies simply because you've been losing on what you're good at, you should stop playing and end your part in the tilt cycle there.