Why don't people use the Desktop much in Linux & macOS to keep their files as they use in Windows?
There are several reasons for this including design choices, how people use linux and which people you have actually seen using Linux, but the most essential one is also the most simple one:
In Windows, if you want a really quick installation of a program and shortcut, the default would be the desktop.
In Linux, the default would be the home directory of your user.
One more fact:
Most people you have probably seen using Linux are advanced users which will generally not use the desktop as a place to store files, because it looks bad and a dock/terminal is better for these sorts of things.
So when you look at it in this way, most users that don't care how Linux looks will not use desktop icons because its not the default, and most linux users that do, will not use it because it sucks. So who's supposed to use it?
I'm afraid that's a question that i won't be able to answer