Star Trek has pretty much built its bones on all species essentially having the basic 'man' shape. According to the Gene Roddenberry franchise, the vast majority of races have the basic one head, two arms, two legs, etc. approach; all that seems to get bandied about are skin, scales, and ridges. Other franchises have mixed it up a bit, but -- in the end -- the economics of presenting life out there (once it comes here) pretty much follows the same approach -- they're gonna take our basic shape and tinker with the appendages.
That's been a big buggaboo for Science Fiction purists down the ages: while our understanding of Science certainly suggests life could (and maybe should) evolve similarly on different worlds, with the elements and evolution of a new environment pushing the form in differing ways. Why haven't we seen more energy creatures or collections of mist and fog? Well, that would require a bigger budget, and Hollywood still prefers putting something on the silver screen within a respectable dollar amount.
In any event, I stumbled upon a small think piece about this question and more on the 90.7 WFME Radio Station site this morning, and I thought it worth sharing. Take a gander when you're on your coffee break; it's a quick read.
As always, thanks for reading ... and live long and prosper!