How do cows become leather?

It does not. Leather cows & beef cows are raised & farmed in polar opposite manners. Leather cows get far more exercise than meat cows which are kept in small pens, so as to keep them fatty & prevent them from acquiring muscle which makes for more tender beef. Leather cows must be muscular so as the toughen their hides. One would be unable to get quality leather & quality beef from the same animal.