Easy.

You can only install it on one brand of overpriced hardware. Does anyone really need to list another restriction to rightfully say that this OS is "too restricted"?

You can't tweak the system nearly as much as you can Linux or Windows. For instance, there is simply no access to certain APIs that Apple uses, such as changing NSScreen visibleFrame, which is why every replacement for the Dock fails with the same bug: Application windows show up behind the Dock replacement.

You can't even change the fucking mouse pointer color!!!!!!! from black to white or any other color that you desire...and there isn't even a third party tool that allows you to do this because of more hidden APIs.

Honestly, I could go on for days about all the restrictive qualities of macOS. I used to have a complete list somewhere, but I chose to simply stop using Macs (unless I have to compile something for iOS) instead of trying to convince others that it's a bad idea.

If you can bear to use it, great for you! Tons of people eat McDonald's too.

In the same line as Ford's 'you can have your car in any color you want as long as it's black', Apple isn't restrictive at all! ...as long as you want to do things the Apple Way™, and only such things as Apple has deigned to allow you to do. Start wandering from the common path much at all and you just straight run in to walls.