Because that's by design. The windows are meant to have different corner radius, they even explained it at WWDC. Then people forgot and rediscovered it again, like it was some new thing.

I am not saying that it's a good idea to have different corner radius, just that it's nothing new.

https://news.ycombinator.com/item?id=47320534

I'm starting to suspect most people at Apple (and Microsoft) just spend time in a browser these days and so they don't notice how the desktop has gone shitty.

Priorities on what tickets to work on, and Apple being proudly underresourced.

Yep, it’s just ugly IMO

Because they did it on purpose to demonstrate their utter contempt for their users and to show us how wrong we are.