Yes, I prefer an Email/password, too, so to depend less on third-parties, and keep it more transparent.
Yet, OAuth/OpenID is significantly easier legally and financially than Email processing (even via outsourced services as MailChimp) and store someone's personal information as Email address in databases, if compared to a social account ID, in long term.
Not only that, but OAuth providers have APIs to get sufficient User information, and regularly actualize, including: Name, Email (yet, by requested/allowed scope only), activity on that social network as posts/channels/followers count etc., which may be a requirement for their Staff/algorithms to determine the priorities for transactions/support and/or security involved.