Well, because it has? Hover over the green stoplight, you get a popover with fullscreen options. Press Option and the fullscreen options mutate into snap left/snap right/zoom. These can also be found in the Window menu. Again, press option to mutate the fullscreen ones to snap. And since these are menu options, one can set any keyboard shortcut of their choosing through the keyboard shortcut prefpane+. So it is not "…
Steve Jobs hated full screen windows. It’s why “maximize” expands based on content and not screen size. I haven’t used a Mac in years but that might still be part of the reason.
This is why Mac apps have a long history of barfing out an irritating flotilla of windows all over the place. Mac OS turns the entire screen into the client area of ONE application, whereas other GUIs can treat the desktop as an infinite workspace.
The welcome move to single-window applications on the Mac has made the blunder of the single menu even more obvious, as users minimize an application and think they're resuming work on the previous one... only to find that the menu is still owned by the application they have dismissed. Absurd and dumb.