Because it's a PSP game and because they're too lazy to animate random civilians.
For a game that has been in development for five years, it makes me sad that they take longer than any other developer [especially from the west] to develop a game with less technical features. I mean, for five years of development on a HANDHELD PLATFORM, I bloody expected a lot of interesting technological achievements to make the game more advanced than it seems with civilians, some interesting shadow or lighting effects or some unique changes to the engine used to have more stuff appearing on screen such as random civilians.
Then again, Mario Galaxy lacked many civilians in a lot of their worlds, yet there was a lot going on, mostly with the environments. It would be great to see KH have more interactive environments which brings the levels to life.
My criticism of Birth by Sleep doesn't stop me from liking it. I'm just kinda miffed that with 5 years of development and a game [Crisis Core and Dissidia] using the same engine hasn't been tailored and edited for KH's world which could boast something more.
One more thing; The whole 'It's a PSP, there's a limit' also makes me sad. If we succumb to that argument, then developers wouldn't be motivated to develop techniques for it. If people say "We can't bring Anti-Alaising into PS3 titles because it does not work well, so we won't', then we wouldn't have MLAA. If we didn't have ID tech saying "Well, we can't make the graphics look good on console and PC using less resources, so we won't", then we not have Rage or Brink which uses the technique which boast great graphics with little resources.
The whole "We can't do much with PSP" is failing the test.