I think you are right. Cloning is expensive, why would dairy farmers do that?
Instead of having a herd with one Betty, one Mary, one Cathy, two Junes, and a Sally who all are different weights, different heights, different lengths and all produce different amounts of milk each day it would be much easier to have a herd of all Bettys. All the same size, same weight and all producing the same amount of milk. The feed differences disappear, the medical requirement differences vanish, etc.
Costwise, once the herd is established it would be cheaper to maintain full production.