I am curious as to why most games do not have a mode in which you can easily move your cursor outside the game window to another monitor for example? World of Warcraft maximized window does this really well. Does anyone have any insight on why this would be difficult and/or not worth it for most games to implement?
Developer priorities. It's more work to support than a simple windowed/fullscreen toggle, the latter of which is more or less built into DirectX/OpenGL and most windowing systems.
There are subtle bugs that you end up finding when you implement it, like on some OSs the mouse pointer position is reported incorrectly, offset by the height of where the title bar would be if it existed. It also requires more in-game interface, since you need the window close and minimize (or at least de-maximize) buttons built into the game's UI.
Many commodity graphics engines do not support it, because it's only useful for some games. That kind of behavior isn't desired in an FPS, for example.
None of this is insurmountable, or even "a lot of work", but it's a measurable amount of developer time to design/implement/test. A simple window/fullscreen toggle in the graphical options is not.
It's not so much a technical issue as a gameplay issue. Namely, if you try to click on something at the edge of the screen but move the mouse too far you might click outside the game instead, making the game lose focus or possibly even get covered up by a window that was partially behind it. It's particularly troublesome for first-person view games.