Having set characters makes them far easier to balance, especially since they intend to keep adding new characters. If you could mix and match abilities the playerbase would know far before the developers what was unbalanced and there'd be a lag in the time it takes for Devs to attempt to balance whatever the latest meta is. This is how a lot of games end up where the devs stop even trying to achieve balance, and instead just get on a merry go round of nerfing the latest meta forcing a new one to appear.
I like the fact that I know what the characters can do - it means that the more you know, the more you know how to counter and play against enemies.
