The key problem is that because these are fairly simple free time projects they are having to emulate all of the graphics, sound, and processing of the console in your processor. This means that for current gen you are way behind in processing power required to emulate.
It just is not at all simple, or even debatably feasible to create an emulator that can utilize your graphics card to any substantial degree. A large part of this is the fact that you would pretty much have to recode the most difficult 2/3s of your emulator for each major graphics chip on the market. It simply is not worth the gargatuan effort required.