Why can't cows sit down?

Not usually. They can't sit on their rears like dogs or cats can because they don't have the agility or flexibility in their hind-quarters like dogs or cats do. It's rare to see a bovine, bull or cow, to sit like a cat or dog can, but it has been seen before.

On the other hand, "sitting" could mean laying down with front and back feet tucked underneath, and the animal laying partly on its side, more onto one side of its hip. This is called laying down, though, not sitting.