Because physically they cannot roll over. They do not have the dexterity nor flexibility like cats or dogs or horses do to roll onto their backs by themselves.
A "cow" is a cow when that "cow" is a she and she has given birth to at least one calf.
Make Mine Milk, a video on VHS released a long time ago.
Stop milking her, or pull the calf off of her, and don't let anything or anyone milk her out for a couple weeks.
cow.