Modern software typically does not have a close or quit function. Android/IOS/Win8 are designed to hold recently used apps suspended in memory instead of closing out entirely.
For example, if you want to stop playing a mobile game, you most likely need to hit your home key or back key a bunch of times, but if you go back to it, it will most likely be where you left off.
Expect the quit feature to become more and more scarce in the future, as it will be a job delegated to the OS.
It seems like a lot of people and game developers are going the mobile route these days. I am not too fond of such a thing, because I am 25 years old and never even really had even a cell phone. I generally don't like the small screens, touch screens, small keyboards, etc, on them either. I prefer to stick to windows games, both making them and playing them. And also playing console games, though making them seems to be outta my league. Most windows games I download usually do have quit options, so for windows at least, I really don't see how such a thing is being phased out, nor do I want it to be. Its quite a basic thing, and it should not be denied to people who want it, especially when its so simple and probably takes little effort to create.