Good question, because I personally prefer human milk myself. Yet for reasons I cannot fathom, obtaining it is much more difficult than when I was an infant. (Even then, I suspect I was raised more on the stuff that could be purchased at the local supermarket if not entirely.) When I try to obtain human milk now, even when I offer to pay for it, I find myself being insulted, threatened, arrested, and sometimes beaten by husbands or significant others.
It actually depends on where you live, in many countries goat milk is used more than cows milk. Cows milk is the best for mass production purposes because one animal can produce multiple gallons of milk per day where a goat can only produce a quart or two.
I don't consume any animal products ~not just milk~ and am finding non-dairy milk is actually becoming very common as an alternative. It is less expensive, depending on the type, and lasts without refrigeration making it last much longer, and be less wasteful by not going bad as often.