Nowadays, a majority of apps require you to sign up with your email or even worse your phone number. If you have a phone number attached to your name, meaning you went to a cell service/phone provider, and you gave them your ID, then no matter what app you use, no matter how private it says it is, it is not private. There is NO exception to this. Your identity is instantly tied to that account.
Signal is not private. I recommend Simplex or another peer to peer onion messaging app. They don’t require email or phone number. So as long as you protect your IP you are anonymous
Just the fact that signal can, and we can assume, does share all the other data outside of the actual message content is a big deal.
You’re just not going to go to the extra effort of requiring a phone number and storing that information if your business model isn’t dependent on selling that information to parties who would want it. That takes a lot more effort than just giving out username/password pairs.
No there is good reason for requiring a phone number, it’s to reduce spam accounts.
Of course they can sell your phone number but that’s not the only good reason for requiring one.