In Ye Olde Days, effects technology really didn't exist to create convincing non-humanoid aliens. Just paint a guy blue or green and call him an alien and let the audience use their imagination.
As effects got better, you got some aliens being less and less human but there's a few problems with this. First of all, money. Effects shots, CGI in particular, costs money. So a recurring character composed of energy or sentient salt or whatever costs money. Shows have finite budgets. So they can blow it on a weird alien, or an awesome space battle. Which will draw more viewers?
Second, it also reduces the actor's ability to act. For a CG effect, you're on stage acting against air. A voice shouts from off screen to say the lines, but you don't have a person to interact with. And so alot of those effects end up with bad acting performances next to them.
Third, audiences don't empathize as well with them. It's great that you thought up this strange thing with 7 legs, 4 heads, and no eyes, but audiences don't connect with it as well.
But we also have to admit that there's an imagination gap. Not all writers are up to the worldbuilding it takes to create new and unique societies as backstory for a weird alien. The more the alien is "like us" but with tweaks, the easier on the writers. Also, if they're like us, but with one or two critical differences, you can use those differences for social commentary. The alien race becomes a "what if" to shine the light on some contemporary issues. If the aliens are just SO WEIRD, then you can't really do that.
Remember, most Hollywood folks really aren't all that bright. They are typically repeating what came before them.