Because everyone will need a (possibly misconfigured) AWS IAM account just to log into any Linux server.. this increases complexity and reduces isolation, compartmentalization, separation of concerns, least privilege, etc.

I was mentioning that particular misfeature because it was a personal annoyance of mine. Oh well, I suppose everything is about customer lock-in these days.

It sounds like you don't think AWS is the most secure place to host an application. That's not the argument being made here; the argument stipulates AWS.