Because that's not what diegetic means. It simply isn't. For something to be diegetic it has to exist for both the characters in the story and the readers/viewers OF that story.
So, again, it's not diegetic if the wheel all of a sudden falls off from the character's car without the player knowing something about how that happened. Because in order for that result to occur, the process has to be visible to the audience. The music stops because the character turns off the radio. We know exactly why the music stopped. Why do we know that? Because it's diegetic. Non-diegetic music usually stops to indicate a new scene of some sort. It doesn't exist for the characters.
Something cannot be diegetic without the audience knowing at least in part, why it occured. It's simply not possible to be diegetic without that information. They don't have to specify it in the definition because it's impossible otherwise.
Can you give me an example of a diegetic scene from a movie or TV show where the audience would be completely without any information about how the results of actions in that scene occurred? I've given several examples of how my definition is required in order for something to be diegetic. How it would be impossible to be diegetic without the audience knowing how the results occurred. You keep claiming that it's not necessary in order to be diegetic, but, so far, have failed to provide any evidence other than a failed reading of the definition of diegetic.