I get it, it's your computer, etc., etc.

The reason this is done is of course that most applications don't have content that fill the entire screen, so maximizing, in most cases, is meaningless - and hinders the usability of the system.

It makes more sense to leave some space over for other apps than have a big empty area on both sides of the screen.

Interestingly, I noticed that it's only Windows-switchers who complain about this. People who've used Mac for a long time don't give this any thought.