Yet, anonymous compute providers exist. Either way, if all you want is to identify who your users are, that's fine. Email hardly helps. It's not hard to setup a temporary mailbox. Anyone can sign up to protonmail without identifying themselves too much. All you're doing is making it inconvenient for the opportunistic an lazy potential criminals (and legit users alike). Simply not accepting crypto payment goes a long way. Furthermore, even if your users are 100% innocent & legitimate, they could and do get compromised (happens a LOT).

I think it's just something people do because "everyone else" is doing it. Lots of familiarity around email. "it's just not done" as they say.