There's a cost to developing features. "Just let me use email" is dismissive to the fact. 99.999999% of users will be able to use one of those. Why spend more time making a subpar email login feature when you can integrate with existing services that probably even increase sign up conversion because it's an easier UX?
Because target users are developers and developers don't like using social logins as much as other users. Using firebase or 0auth is pretty straightforward as well.