Sure! I agree. There are plenty of different authentication standards. But if these are all highly ubiquitous standards implemented by a myriad of apps, why is

Sure! I agree. There are plenty of different authentication standards. But if these are all highly ubiquitous standards implemented by a myriad of apps, why is this work passed on to the developer, and not handled by the operating system? Why can't I pass my OAuth tokens to this hypothetical login view (a la Firebase Auth), and let that handle signing my user in through X service? Why can't I just provide a shibboleth endpoint and let the native view handle exchanging tokens? I'm not asking Apple or Google to figure out my highly custom one-and-done authentication protocol that relies on smoke signals and written poems to let the user sign in. I'm asking them to give me the tools to support these decades-old standards, standards that basically every app has to support already. It makes sense especially in the context of GDPR to provide system-wide support to a legal problem, as opposed to each developer having to solve the same exact problem over and over again.