It can't do the literal entire thing an operating system is supposed to do: manage applications and their resulting windows, in a sensible way.
I want to know what application is running.
Sure it's in the dock!
I want to find a specific application window.
Go fuck yourself right to hell.
Wait, the taskbar doesn't show the running windows, like it does on every other OS? It's at least discrete right?
It discretely takes up 1.5cm of the bottom of the screen at all times. It's so discrete it doesn't even need to use the corners.
Uh, alright, well that's all the system space you need right?
Yeah of course just that bottom inch or so .... And a top of screen system level menu bar to display what windows does in the bottom corners.
/sigh/ ok, fine, I just want to be able to full screen a window and still see what else is open.
Burn in hell and die.
I want to be able to easily switch left and right between open windows.
Go full screen or I will shoot you.
I want to move an open window into the other monitor.
You can't because you're full screen dumbass.
I want to let a window present a popup like they normally do.
You can't because youre full screen dumbass. Why would you be full screen?
I want an application like Slack to be able to popup and remove notifications when is appropriate.
Choose to have every single notification persists on screen until you manually remove it, or miss all your notifications.
Can't we trouble you for something in between, where we trust an application and let it manage them in a way that makes sense based on their context?
You can trouble me for something in between these cheeks, shit stain.
Like honestly, I fucking hate what an advertising and AI filled mess Windows is, but it can actually manage your windows and virtual desktops in a way that makes a modicum of sense.
It feels like a single Apple product manager decided that the way that they use their computer (a single application at a time, no windows to manage) is the only way anyone does, so who cares if we implement a nonsensical full screen paradigm, it makes one tiny niche edge case slightly simpler.

“It’s not a dick measuring contest,” but also: How many windows you have open? Bro, how many?!
Really my biggest gripe with iOS/MacOS is that a lot of functionality is designed for an apparent fluidity rather than intuition — I find you often have to be taught how to do the things you are complaining about.
Stop. Just stop.
Look through the comments. Literally everyone rushing in here to defend Apple says something along the lines of "skill issue" or you just need to be taught how to do it. But when I prompt them to explain how they switch to a different window, of the same application, on one monitor, they list a convulsed series of steps that are decidedly not:
"Alt + Tab"
or
"Three finger swipe left and right"
like they are on windows / decent Linux desktop environments.
Apple's full screen paradigm has always been nonsense. They pushed it super hard for every application because they have a shitty dock that takes up too much screen real estate, and it's always caused problems with the entire rest of their windowing system as a result.
You know what is fluid? A three finger swipe left and right with handy window previews. MacOS's window management is just UX band aid on top of UX band aid, but they market it as innovation.
Until recently I felt Apple UI were pretty intuitive actually.
IpadOS is an abomination unto the lord though.