Do college coaches call because a) they don't have experienced catchers or, b) they think they can do a better job regardless?
College coaches call pitches because it is their livelihood on the line and it is hard to put that in another persons hands.
Same for MLB coaches. Lots of travel coaches do it because they see these coaches do it.
But I'd say most coaches do it because being head coach is a job for control freaks any way.
I'd love to see it limited some way especially in baseball. It is a big contributor for the games taking so long.
As a head coach it depends on who my catcher is. For inexperienced/ones I don't trust I call pitches and have a 'your call' sign which is one of the ways I try to get them experience calling games. Some the signs are more suggestions (and I actually have a sign for 'not a choice - throw this'). A few earned their way up to calling the whole game and man were they a pleasure to coach