I have been looking into this recently and it seems that a lot of sites are using OAuth2 over OpenID. Even when they are using it as a simple authentication me

I have been looking into this recently and it seems that a lot of sites are using OAuth2 over OpenID. Even when they are using it as a simple authentication mechanism (The scenario that lots of "OAuth2 vs OpenID" posts, blogs and tutorials say is where you should be using OpenID ) Since this site is also using OAuth2 instead of OpenID can I ask for your reasoning?