They only provide Google / Facebook buttons because they are by far the most popular option. In practice the vast majority of major online web services support some from of openid / oauth signon, from Github to Steam to Wordpress and more.

As a site developer, it is your job to figure out what buttons your user demographic will use. If it is all facebook / google, then provide those, and most of these sites know providing more obscure options only confuses their potential users.