I guess a super obvious question is, why do they do this instead of having a robust antivirus ecosystem?

I mean I guess I already know the answer, "marketing". "Look, macOS doesn't require antivirus!"

Personally I don't want Apple verifying or revoking anything. I bought the computer, it's mine. You don't get to tell me what I can run, period. Inform me, sure, give me links to go learn why you don't want me to run something, sure. Don't prevent me from choosing to do with my machine what I want.