The reason: Apple designed OS X to focus on using multiple windows at once, and they didn't want developers making single-window apps that ate at the screen. It's why, of their own applications, the only ones that allow full-screen are their video players, the iWork suite, and the pro apps. Those are the only applications Apple thinks ought to be maximized.
There's an application called Mega Zoomer that gives all Cocoa apps a sexy fullscreen view. But mostly, the answer's that Mac users don't like fullscreen.
But they do have a choice, thanks to the aforementioned app. I have it, yet I'm still using only half my screen for Safari, despite no other apps being open. I like not being forced into this maxed-or-not mentality; on Windows, it was too easy to make every app take up the whole screen, and tab over. On the Mac, I've never gotten into that mindset, which I love.
Actually this kind of reasoning was the primary reason I turned away from Mac.
It's pretty obvious that you don't have a choice and that Apple, right or wrong, has mandated that you don't need to have your windows in fullscreen mode. Instead of openly discussing the shortcomings of the OS you come up with this sort of reasoning: I really don't like being forced into maxing my windows, much better not to have the choice. You don't need it. Besides if you really want it it's easy - you just go to site xyz and download the app that makes it possible. To me that just a semi-circular argument, and not a very good solution to a problem that I'm sure a lot of people have.
There are many examples like this, and the reasoning is always the same: You don't need that, it's much better to do it the Mac way. We know much better than you what you want....
Sorry to be harsh, and it's not picking at you Zimbabwe, it's picking at the Mac cult.
Don't apologize for harshness: Your view's completely valid. I prefer a system that's that locked in, because it means I can figure out the computer's philosophy once and then everything gets easier. (The Mac's incredible dragging and spring-loading systems are the best instance of lack of choice making things supereasy.) But that's a personal choice, and one that definitely turns some people away.
If I may say so, you fail to see the forest for the trees. Windows and Mac are both in the same boat. They decide what is good for the user, they are closed source, and then you have to write software to do trivial stuff.
I prefer GNU/Linux, it allows me to change what I need to.
You have to write software to do anything, anywhere. Strictly speaking, with Linux you need to write software just to have a GUI. All of the "trivial things" you can do with your computer are things people wrote software to allow you to do.
OS X is completely Unix-based. All the things you can tweak are things I can tweak, too. I can also treat OS X like Linux if I want to: Getting Gnome or KDE to run on a Mac is really easy. I tried it out by downloading Amarok, which was a huge mistake because the lack of polish in KDE shines through when it's next to Mac stuff.
(I can make an argument about why closed source is a bad thing, but it would be rather lengthy. I'm assuming you'd rather me not derail this thread by providing it?)
There's an application called Mega Zoomer that gives all Cocoa apps a sexy fullscreen view. But mostly, the answer's that Mac users don't like fullscreen.