Why is keeping your coin on exchanges dumb?

I see folks say this here and on the sub CC…. Is it just because they’re younger companies? Because of shady shit they’ve done? Because of weaker security? Help me understand.. because if crypto is going to go mainstream it’s going to need to get to the point where leaving my crypto on the exchange is not any more dumb than me leaving my fiat with my bank, or stocks with Charles Schwab (or the like). What am I missing? Thanks!

EDIT: thanks all for the insight, but do you think I’m wrong that at some point if crypto is to really be mainstream these exchanges will need to be similarly safe as stocks left in the custody of your brokerage house? Thanks again!