Companies like google, facebook, and apple typically have better security. Other companies know that so rather than contracting with another third party or implementing themselves, they use oauth.

With oauth, apple, google, microsoft etc. will vouch for you. There are advantages and drawbacks, with, imo, the drawbacks outweighing the benefits. Key benefit being better security over poor practices and convenience. Drawbacks being less control of your accounts, consolidating your credentials into one basket, (especially if you use weak authorization), and the potential (likely) situation those accounts are monitored