1. Presumably they didn't originally domesticate cattle for milk, they domesticated them for meat.
2. As has already been noted, early humans could digest milk products, just not raw milk -- cheese and butter and fermented milk (tastes nasty but will keep you alive) were always edible.
OK, lessee if I've got this right: humans, who already had domesticated cattle for meat, cheese, butter and fermented milk, selected a gene that would allow them to digest whole cow milk, in order to have better nutrition, by being around milk. Not drinking it after a certain age, just by being around it. And this tolerance for that dairy product, unlike those other dairy products, made them candidates for survival, while those who... didn't stand close enough to the whole milk... died before reproducing. It makes perfect sense. Those creationist idiots will probably make fun of that logic.
Wild cattle were being caught, corraled and milked long before it occurred to anyone to domesticate them.