Like someone said earlier: because Mac OS has always been window-centric, while Windows has always been program-centric. Apple’s philosophy behind their design is that most of the time, you want to be able to see several windows at the same time and to be able to click through to any one of them.
If you change a window to full-size when you first open it, most programs will make that the default when you open a new window. The Finder sticks with the last change you made to the window states when you open a particular location. I will admit that I wish the behavior was more consistent. Pre-OS X, the behavior was rock-solid consistent. Now, it depends a bit on the program. Most of the time it makes sense, though.
Using the Windows taskbar is not anywhere near as friendly to me as the dock is. I intensely dislike the way the taskbar in Windows groups things, and they way it truncates labels when you have several things running. I have to click through or hover on a few buttons before I can find the one I’m looking for because there’s nothing to tell me which of several buttons it is. I also don’t like having to go to the bottom of the screen to get to the task bar to get to another window. Directly clicking on a window I can see behind my current one is a lot faster than going to the bottom of the screen every time. If I don’t want to manually resize every window instead of maximizing, I have to accept that Windows will take away the option to have multiple windows accessible.
I like that clicking the green button sizes the window to whatever it contains. In contrast to the taskbar’s weird grouping and truncated text, the dock has miniaturized versions of the window as it actually appears, giving you a clue as to what it is even before you get to the hover-text. I like being able to see at a glance what window I’m going to open. Expose by itself, or in combination with the command-tab, lets me find whatever open window I’m looking for very quickly. That’s assuming I don’t just have the windows overlapping so that I can just click on one to bring it forward. I usually run with several program windows open at once, with sometimes dozens of others lined up in the dock.
threemae, you can stuff your Koolaid comments. There must be some reason you’re using OS X in the first place. If you don’t like the way OS X works, there are mods out there that might make things nicer for you. Or you can stick to Windows. Complaining about they way something works is fine, but different people prefer to work in different ways, and you shouldn’t imply that they are deluded for preferring that method. Some people really like Apple’s software design, and the fact that they have interface guidelines while Microsoft, to my knowledge, does not, might tell you something about how much those two companies consider the end-user in designing their products.
I can understand that you might not like the way it works, but I don’t like the way Windows works a lot of the time either. Windows drives me nuts when every single app wants to fill the whole screen. It feels claustrophobic and often gets in the way when I want to be able to see a few different windows at the same time. I currently use both Windows XP and OS X, but I really prefer OS X for almost everything.