Because the modes are casual, I should be able to have a bit of fun, some games will be easy, some games will be harder. I can experiment with different styles, loadouts whatever. SBMM in casual modes ruins this, because every game turns into a sweaty slog.
You also dont improve at games past a certain point by killing bots and players just as bad as yourself. I got better by playing against and with the better players, the lobbies were more diverse which was more fun and better for learning.
Play ranked for SBMM, casual mode lobbies should be random and diverse.