The thing is, if Stealth Rocks was put at a set amount, then there would be no difference with it from Spikes other than the fact that it hits airborne pokemon and doesn't stack. I guess that's better than Yanmega and crew getting half their health taken down on the switch in, though.
Of course there would be a difference. You can lay multiple Spikes.
Or for an even more snide answer;
SO WHAT?
Lots of different moves and abilities have almost the exact same function.
EDIT: Hell, even moves like Seismic Toss and Dragon Rage can't be supereffective.