Usually they have an in universe explanation that humanoids are all derived from some progenitor species that was humanoid in shape - but for TV shows and movies it also makes it way cheaper and easier to film historically.

In scifi novels they'll get weird with it. Sometimes you make contact with living EM radiation that eat stars bit by bit or a pool of living water or life that evolved on Jovian planets that float or sink or Silicon based life that respiratory out blocks of sand - they can have totally different conceptions of morality or what is sentience. And this is going way back even back to the Golden Age. The Lensmen Series had some really alien aliens but you kind of forget I guess as universal communication makes it easier to relate. The 3 Body Problem series has some really weird out there biology that, mind the spoilers I guess, imply and later outright state the universe and laws of physics even the number of spatial dimensions were all products of living sentient creatures and societies.

It can also be hard to do the world building of hundreds of planets with hundreds of ecosystems and mostly independent biological histories - easier to handwave and not really get into the nitty gritty if you're doing stuff on an epic galactic or universal scale. But, some people like the world building part! You can check out Snaiad or Barlowe's Darwin IV in Expedition for some interesting speculative biology. Veldhuizen's The Cluster has some interesting speculative biology on plenty of planets.

In terms of pure biology, bilateral symmetry seems fairly successful but that could certainly be a quirk of our planet. Ravens, Humans, Pigs, Parrots, Octopuses, Bonobos, all have bilateral symmetry and are at least moderately intelligent - which leads to one mouth for the digestive tract and two eyes that help resolve depth, two ears that do the same for sound, etc. This is getting in the weeds, but personally I wouldn't say humanity's particular version of tool using/technological society forming intelligence is any guaranteefor evolution - naturally we tend to think so, but if you consider numbers of genomes or amount of carbon used in biological processes (both rough estimates), evolution by natural selection seems to settle on bacteria and plants as it's most successful preferred form of life. In terms of Earth's actual history, life formed pretty much as soon as water was on the surface, anything we'd recognize as complex animal life came much later and humanity is barely a wink in evolutions history on Earth. I wouldn't be surprised if we are it in our entire galaxy even if life is particularly abundant (and probably mostly slime balls with algae and plants lol).