Why do horses and cows have a lot of molars?

Answer 1

Cows have evolved to have only lower incisors based on the forage they eat - grass, sticks, weeds and other tough forage. Instead of upper incisors cattle have a tough dental pad. This allows the cattle to break off some of the vegetation they consume but allows them to rip up some other plants so they also consume the minerals found in the soil on the roots of the ripped-up plants.

Answer 2

Horses and cows are both herbivores, meaning their diets consist primarily of vegetation - grass, leaves, weeds, etc. Plants have very thick cell walls for each cell that is strongly attached to all the other cells around it - this is why grass stems can grow to 18" long or longer. To break down the plant fiber, horses and cattle chew their food very thoroughly and grind it in their large molars.

Interestingly, though, horses and cows don't have any more molars than humans - 3 in each half of both jaws. They are just very large and have deep grooves in them.
Because of the food that they eat needs a lot of chewing.

The food that they eat is harder to digest, so they have to chew more than us humans.