Because it's seen as cheaper to develop a single "one size fits all" solution by those who DON'T FLY THE THINGS FOR A LIVING.

As an aside, this is why I still have separate digital camera, cellular phone, and MP3 player. Can I get away with one single device? Yes. Will it perform all functions equally well? NEVER, or if it does, all of them equally BADLY. The digital camera video functions and MP3 player will sap the phone's battery like nobody's business (making it useless in an emergency, because you can never anticipate when those will occur), the camera will always be compromised, even if it's simply the style of lens they use with it (fixed .0625" distance lenses are inferior even to the extending lenses of cheap p&s cameras), the MP3 player will have to be controlled by either touch screen, or a set of buttons I can barely tell what does what by touch because it's a phone keypad (so forget doing it quickly when I need to kill the music)...

OK, yeah, I feel pretty strongly about combined devices of disparate uses.

The literal only way they could make it worse would be to try to make it a fighter plane for all roles, and a VERY LONG RANGE HEAVY BOMBER THAT DOESN'T NEED AN AIRCRAFT CARRIER TO DO THE JOB.