Lack of imagination
Whenever I watch any Star Trek series it seems like all the aliens have bodies just like ours. And the only difference is the head and face. And usually that only means ridges on their foreheads.
Wouldn't there be more variation in humanoids from different worlds? Yes, they have different insides when examined by sickbay. But on the outside only their faces and hair look different.
Seems to me that is an indication of a lack of imagination or financial means. (As in they can afford some facial makeup and maybe some facial prosthetics. But anything more would be too expensive.)