You can buy colour stuff separately (and that's how labs do it, 50L+ at a time) but there are so many components that the manufacturers tend to sell kits. It's the easy way to make sure you aren't missing a component.

A proper C41 process requires a developer (3 concentrates), bleach, fixer and stabiliser. That's 6 different concentrates you need to mix up into 4 baths, which means that buying separately is annoying, especially if you don't know exactly what you need. E6 is slightly more complicated again.