Yeah, in Star Trek they pleaded panspermia as the reason why all aliens were humanoid. I preferred Babylon 5. The reason is technical, not just a matter of identification. It's harder to make monsters work, and the result may look more like the moppet show than SF.

Nowadays, with CGI, everything is possible, but still George Lucas opts for Jar Jar Binks. Yuck. Well, Lucas does have some pretty "different" aliens, so I guess I can forgive Jar Jar, at least this facet of him.

I am not sure aliens would be that difficult to understand. I think alien evolution would work along similar lines - altruism vs. self-interest, etc., so there would be recognisable traits. There would be differences. Like lack of sense of humor.

Science fiction books have two points of interest. First, there is a plot, which is basically the same in science fiction, fantasy, or contemporary novels. It can be interesting or not, it can depict interesting relationships or not, it can develop philosophical problems or not, it doesn't matter which genre of fiction it is.

The other issue is science and technology and the way it affects society. Things like Frank Herbert's "White Plague", where you introduce a little virus, and see what it does to the various cultures on Earth. But sometimes this science thing is just part of the setting, and the whole thing could be transformed into a contemporary setting. For example, the main theme in Dune is the scarcity of Melange, which makes the planet dune very important. This is science fiction, but you could easily transform it to a contemporary novel about scarcity of oil and the importance of the oil producing countries. Suddenly Emperor Shaddam IV becomes George Bush or whatever...

Dressing it up as "Long long time ago in a Galaxy far far away" helps you as an author to add some additional sub-plots. Fantasy could serve the same purpose. It also helps putting some wishful thinking into your novels - no traffic jams, global governments, magic, dragons, whatever.