Part of the problem is that communities can get really nasty, worst case going as far as writing bad reviews at the place you work at. So it's useful to containerize your identity.
Reddit actually does this very well.
Say for example that your “real” iCloud account is muzani at iCloud dot com.
You browse to my hypothetical forum and choose Sign in with Apple. Then Apple lets you choose if you want to use your “real” mail address or if you want to make a new alias specific to this service. You make an alias an it will look something like emperor_virginia.9p at iCloud dot com.
It’s super convenient and easy to use, while at the same time doing a great job of keeping your identities separate between separate services.
I have no idea how long these random companies will keep providing auth services, I don't want to be part of the mess when they inevitably stop the service or start charging. It barely solves the account fragmentation problem because every website supports a different random combination of providers. It centralizes everything more than it already is, further increasing your dependence on these companies. Not to mention the tracking.
But the biggest thing is that with a native account I know that I'll be getting all the features of the app and everything will work normally. There have been too many times where I've experienced "your account does not support this feature" because of a botched external auth integration. Remember when Spotify users with Facebook sign-in couldn't change their name from the default string of random characters?
No thanks, the "effort" is more than worth it for me.
What if the forum offered the following three options for sign in:
- Sign in with GitHub
- Sign in with Apple
- Sign in with Google