Why do cows moo not roar?

Because they can. It's a form of vocalization that tells other animals (other cattle, humans, etc.) whether they are looking for a lost friend or calf, hungry, ready for battle, in pain, dying, in heat and looking for a mate, etc.

But really, in real life, cows don't make a "moo" noise. It's more of a "mmmrraaww" noise. The description of the noise a cow makes in the English language is the best representation in as few letters as possible that we can make.