The games are coded to work with touches on specific parts of the screen using a virtual joypad. This sort of thing is very hard to emulate using the accelerometer and is very dependent upon the game itself since all of them have different virtual joypads.

You'll find that most native games with accelerometers already work (eg. Need for Speed) but will be almost impossible to do a generic one to make it work for every other game.

Mupen64 is different because of the defined keys on a physical keyboard that is much easier to map to accelerometer settings - unlike specific locations on the screen.