Security/privacy yeah. I don't do business with Twitter/Facebook for the same reason. In the case of GitHub, if I want to contribute something, I am going to do it volitionally, knowing they will do whatever they want with it.

Creating an account just locks you in, when the alternative exists or existed before. SourceHut proves this is possible. Why not allow non-accounts to contribute?

My GitHub account is blocked because I refuse to give them my phone number or install their app. They refused a GDPR deletion request too.

OK, that's a really solid reason. Thank you! For me, it suggests that someone like you might not mind creating an account on a system I run (because I allow anonymous account creation without anything at all although providing an email is encouraged).