Honestly? Because fighting school can be fun, it's a simple as crap shonen setting and one I like. But when you turn it into powerwank central where characters need the ability to effortlessly instakill anyone that doesn't have some means of heightened defense to survive, let alone remain competitive, then it stops being as fun, because everyone tends to go for ridiculous powers in an effort to not be shown up by others. Then rather than a fun game about fighting and shenanigans it turns into everyone having a nuke button. 

It's "fine" when it happens in bad shonen because there's a single writer controlling the narrative and he doesn't need things to remain balanced and reasonable. In a roleplay it just turns into a stupid arms race where people strive to be as overpowered as the GM will allow to counter or at least balance out with the other characters. But hey, don't mind me, it's up to the GM to judge and people should just follow whatever powerlevel's allowed.