"why this is not an extension" sounds like an awfully naive question. I'm not a Godot expert but I'd bet a very large amount of money that this is not in the realm of a simple extension, as flexible as Godot can be (and a check of the PR seems to confirm this)
With Godot, you can’t make sweeping platform changes—especially adding a new one—as an extension. This is the only path besides Apple forking Godot, which would be unworkable in the long run and obviously undesirable for both parties.
I'm glad some people like https://github.com/godotengine/godot/pull/105628#issuecommen... exists
"why this is not an extension" sounds like an awfully naive question. I'm not a Godot expert but I'd bet a very large amount of money that this is not in the realm of a simple extension, as flexible as Godot can be (and a check of the PR seems to confirm this)