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.
because they are cows not lions
Uh, no...cows do, gorillas beat their chest and roar.
"Moo cows" are, essentially, cows that moo.
yes all cows moo.
To the moo-vies! UK cows go to Uddersfield it likes to eat moo-slie at the moo-vies
Yes.
No.
moo moo cows that are weird
They have a stuttering problem
Yes but only if its loud moo
No, not usually.
Yes.
Frogs don't moo because they aren't cows!