It's not that weird. Building a modern accounts system is a lot of work and people don't like creating new accounts. So it's the path of least resistance for developers and users.
It's not even that it's hard to build a modern account system. It's that if you put your site behind a private / custom account system, and you post a link to it on HN (for example), everyone and their mother complains that it's requiring an email address and password. But if you put the same site behind a Google auth, most people wouldn't think twice to click the button.
I dunno, I’d say it’s never been easier.
If you’re building a paid service you’re probably already using cognito or supabase or something. That puts you a few clicks aways from 5+ other identity providers and normal accounts.