You could try Home Depot, I hear they sell paint by the gallon.
Joking aside, that's really a good question. I'm thinking that besides a few colors, people don't go through the paint that fast. With the little bottles, if the little bottle dries up, then it's not a big loss. Or if it spills, less of a mess.
Also, there's probably a consideration of space. With all the different colors, the larger the bottles, the more space we'd need to store them all. I know I easily have 30+ bottles of paint. Then there's probably cost issues. Easier for companies to manufacturer and distribute one size of paint vs. a couple of different sizes of paint bottles. The hobby stores probably are at a premium for shelf space.
Hmm....guess I was babbling. I hope there's an actual valid point in there somewhere.