I mean it's also that these aliens aren't meant to be alien but rather representatives of different human things. Star Trek especially, most of its aliens are explorations of historical human cultures and ideologies if they had advanced further technologically and been pushed to certain extremes or so on. When Star Trek does want to do the truly alien it does things like the original series episode about the sentient rock slug thing. Another example would be War of the Worlds, where the martians are meant to be colonial western powers but even more advanced and further evolved by a few hundred years. So they're humans but with bigger brains and hands to emphasise how they've primarily been interfacing with tools, with the explicit theory in text that we'd evolve in the same way over time. And the things they do is just colonialism and it's not trying to be subtle. Keeping the aliens somewhat human is what allows the political commentary to have some bite to it, if they were truly fully alien it would suggest that these things aren't human traits and beliefs, because the truly alien is defined in its distinction from humanity, but by keeping them somewhat human it suggests that there is some similarity between us and that we ought to consider it. Obviously the empathy and budget constraints are there too, the point of human aliens is ultimately that they are better suited to whatever themes the work is trying to explore. The truly alien in fiction has a different role to it and is used as such, and you can explore these themes with it to an extent, but the way you would do it, and with how much depth, would be wildly different.