As stated before, they choose the highest security possible, and Discord as a service defines that as tying a phone to the account.
You can be upset all you want, but it is by definition, Discord requesting your phone number as a neutral party in the interests of both you and the server in order to be what Discord has defined as the most secure method of positive identification.
You don't have to like it, and there's nothing saying that the admins like forcing phone numbers to accounts to be present in their server, they've only deemed it necessary to ward off problematic accounts. The community they run is theirs and that decision lies in their hands.
Are there still questions about this?