I could stop trying to use Discord and drive to Best Buy and buy a cell phone and pay for a month of service. Then I could add the number to the account. Then if I stop paying for the monthly service, there’s a good chance that Discord or whoever won’t believe I’m me at some future login and will demand I give them a code they sent to the phone number on file.
That's a lot of effort/cost that the user of a website/server has to do, just to access that website/service, when email validation already works (not as well, but still works).
The onus of the effort/difficulty of dealing with bots/etc. has been shifted from the website/server owner, to the user base.