I'd like to think that too, but...
Milking isn't that fun. It's degrading, humiliating, kind of a pain, sort of a hassle, etc. I'm not saying all that's actually true, but that's definitely how a lot of people would see it.
Now think of some Boobhus. Yukari, Ran, Yuuka, Kanako, Kasen, Patchouli, could you see these sorts of people all hiking down to the local milking station to do their somewhat shameful public service? I could not.
That's the the problem. I'm sure we could count on folks like Keine, Byakuren, maybe Sanae, but that's not enough. In order to reach Gensokyo's quota, we need ALL boobhus doing this, or at least most of them, and I'm not sure that's feasible.