Or maybe the makeup's just cheaper for humanoids.
The early episodes of "Star Trek TNG" extensively used what I called the "Silly Putty" alien creation technique. Slap some Silly Putty on the forehead, you're one kind of alien. Slap it somewhere else on the forehead, you're another. Put on on the neck, the cheeks, whereever.
This improved a lot when they got some computer graphics capacity under their belts.