Speaking as a developer - the reason that some devs choose to use Google/Facebook for sign-in/account creation is because spending the time to build a user account system is not only a pain in the butt to do right - ie in a manner that doesn’t result in your username/password being plastered all over the internet - it’s also time wasted on developing functionality that other people have already created. Devs don’t want to spend time working on the dreary stuff like that, they want to be working on the fun, actual functionality of the app.
Edited to add: on top of that, a WHOLE LOT of people are very happy to have less accounts and passwords to remember, as well, so removing barriers to entry is a big deal too. Just look at this thread here on the BBS talking about how there are (a little bit) fewer comments nowadays, due to people not wanting to bother to create an account (which in the case of internet commenting, is definitely a net benefit. You get far fewer drive by trollies).
I personally would prefer something like OpenID, myself, but I can totally understand why the developers would choose to go this route.
