It does disrupt the user experience and clobber a site's own home page, but associating a persistent identity to a user is VERY valuable to sites, and normal sign up flows have a lot of friction. They probably figured it's worth the cost if it gets more users to login/create accounts. Its presence despite the drawbacks indicates a lot of people use this feature.
I don't think so. Far as I can remember Reddit decided to lock down their site and start monetizing user data. They don't and never have had any sort of widely used authentication like googles SSO and Email combo. Far as I can see Meta and Google are the main "Real users" players at least in the wider internet ad markets.
Actual Identity companies have almost no presence outside of banking/finance/gov from what I see.
