All communication with professors has to be official over here to prevent abuse. Phone messages don't leave a trail that can be investigated, emails do.
Plus I'm pretty sure you could insist on getting a phone number from your university if you didn't want to share yours with them or if you didn't even have one, for some reason. That would just lead to an enormous hassle for the university over here, it's easier to just give every student an email address.
Well, at least not in the same way as email with a centralized server.
Unlike email, most modern messaging services are end-to-end encrypted. If an abusive professor manages to manipulate their victim into deleting the messages (and does so as well) it's nigh impossible to verify accusations. With email, the university's email server keeps them stored and visible.
Unencrypted messaging poses severe privacy risks since - you know - the messaging service can read every single message.
Usually I am all for encryption, though for official communication I don't think it's the best idea. It does have the minor drawback that particularly sensitive information isn't sent (such as exam scores) but you can just check them on the university's web portal anyways.
"oops I just sent a dick pic gallery to the whole faculty in the 800 building 😳 I hope they won't call me in for a group meeting tonight to reprimand me.... unless... 👉👈"