"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)
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)