The real issue is not mechs having heads. It's the top humanoid design itself. Why make a war machine top heavy and having critical failure point of losing any of the joints in one leg and limbs that demand thinner armor, etc. When you make a humanoid mech or some cases a mech at all (I mean any kind of walker with any kind of bodyplan) you have already entered the "stupid n improbable territory" so why even bother caring about why it has head or not, if your aesthetics sense demands mechs.