They probably do it to prevents spam/abuse. It is supposed to be a better WhatsApp after all, not a completely federated software. So it gotta be somewhat user friendly.

How else are they going to track you?

No, seriously. Even if the messages are encrypted, the metadata including your account info and the account info of everyone you talk to are not. In a lot of these cases, they don't have to have the actual contents of the messages to have a pretty clear picture of what you might be talking about!

With a phone number that's almost certainly registered to your real identity, it makes it trivial to track what you as a person is doing even without breaking the encryption! An encrypted messenger that requires anything related to your real identity to get an account is security theatre.

For example: if you suddenly start messaging back and fourth with an account, and that account happens to have the same phone number as the one on the business card and website of an out of state abortion clinic worker, and your own phone number's area code just so happens to fall in a state that banned abortions after Roe v Wade got trashed, it juuuust might imply a few things about you. They can't definitively prove what the messages were, but if your state criminalizes any and all attempts to get an abortion anywhere, it's probably enough to get a warrant against you.