Doesn't Google already penalise websites for poor performance though? Why not just intensify that penalty, rather than develop and promote a new framework intended to forcibly prohibit bloat?

Sure but people won't always respond to incentives. It's like asking why AA exists when the cops will already throw you in jail for being drunk in public.

Google will rank results partially based on page performance and behavior. It is possible to improve your ranking by improving page experience. AMP is the complement: a tech stack that makes it impossible to not do those things.

When nearly every site has horrible performance IDK if it would make a difference to intensify those penalties, as they'd apply about equally for instance to every news site, every blog, every e-commerce site, etc.

That was the replacement mechanism after AMP, yes