Because I'm just a lowly ROM hacker, and not a very good one at that.Though I was thinking at one point that it'd be cool to make my own NES game. Though I'd either have to find someone who knows 6502 ASM code, or learn it myself.
There are some SNES homebrew, but not much. There is a cool looking game coming out for SNES by Watermelon Team(whenever that actually happens) Though really there's way more homebrew on the NES than the Genesis.