Of course, the real reason why Star Trek aliens look so much like humans is because of budgetary practicalities. It can cost up to $200 to apply a simple nose prosthetic to a Star Trek extra. Full Cardassian makeup can add $2000 to the budget per day, and they’re still humanoid bipeds with a standard number of eyes. Unless there was a compelling story reason, truly alien aliens were seldom seen in the Star Trek universe.
— Star Trek: The Next Generation: The Continuing Mission, by Judith and Garfield Reeves-Stevens