When you see a button on a webpage, you probably know what it does. If you don’t, the page is poorly made. But just because you do, when you see it, doesn’t mean a disabled user does when interacting with it.
Google is not infallible. Even they fail at following accessibility best-practices. I have found 3 examples of (very common) mistakes, that even the bright minds at Google and all their efforts and reviews failed to see before shipping them to end-users.