We're not herbivores (your teeth are one of the best indicators here, no herbivores have front teeth like ours). The main reason we don't have enzymes like cows is because there was no evolutionary pressure for us to, our ancestors have been able to get by on more easily digested vegetation and meat.

On a similar vein, the reason we can't eat raw meat like other meat-eating animals is because digesting it takes enzymes that we lack, which we probably lost because they take energy to produce so our ancestors eating cooked meat could survive more easily without them.