This is mainly an UI issue. Browser vendors were disincentivised to offer straight authentication API both easy to implement for websites and easy to use for users, because it was seen as threatening to users' privacy.

Maybe it is harder than I think, like, why can't the browser maintain an identity/profile and allow me to use it to one-click sign in to any website? But I don't see anyone really trying, only saved passwords hacks. Or it is supplied by Facebook and Google instead of your browser.