SSO can seem intimidating to someone new to authentication implementations. I think this is in part because it is a bit complicated but also because it is an inherently invisible technology which we dont need think about as users. Sign in with fb/google on the other hand is something we all see in our day to day business and have at least some kind of mental picture of how it works. We've also seen it used as authentication for otherwise poorly implemented services so we might think that "since they managed to use it, how hard can it be?". Hence, a lot of developers and product owners choose those instead of SSO.