If people would go to a dairy farm they would realize that its impossible to milk anything but a cow there.
One you cannot easily milk a pig, Unlike cows that are quiet when they are milked, Pigs are very restless animals and will not stand still while being milked and more importantly, NOBODY milks a cow anymore, its all done by machine with these pipes that fit on the cows udders, and machines suck the milk out of the cow, The pipes do not fit on a pig and lastly even if somehow you could milk a pig (It obviously has been done) the milk of a pig is worth more than milk from a cow, there is no financial incentive to do so