One word: battery.
It's the reason why I have an Ipod, DS and a cell phone. Last thing you want is to miss that important call because you played too much monkeyball. Videos are killing the battery quickly on the Ipod as well so I don't see much use to play video and video games on the same device. If they would give us a device that has 100 hours of battery life playing videos, it would be different.
How many cups of darkness have I drank over the years? Even I don't know...