It is sincerely impressive Marvel themselves have not done a classic sentinel but with a more feminine shape in the comics yet. Incredible restraint, really (Prime and Omega sentinels notwithstanding, I mean a big 100ft tall purple and red version).
Y'know, thinking this way, there's also something to the whole ethos of Sentinels int heir purpose. They're built to wipe out anything that isn't mankind, so mankind built them intentionally in its own image almost as a cultural statement. "It may be metal, but it's the will of a person that is killing you" kind of deal. A drone or a mini-tank or any other, more efficient body-plan would remove this key element. It's not just killing mutants literally, but as a symbol as well.
I mean, of course the writers and artists didn't make them look like humans for that reason, but as a no-prize explanation I think it works, and kind of makes them feel even more sinister.