Why Should Aliens Be Humanoid In Appearance?

For the sake of argument, let's just say aliens do exist in the forms that get reported by people who claim to have seen them. A few possible reasons why they almost all seem to be of a humanoid shape, whether reptilian, gray, Nordic-type, etc.:

1. Our evolution on Earth has been guided for a long (perhaps extremely long) time so that we have taken forms deemed best by whatever beings are controlling this process. David Brin's great science fiction series called the "Uplift"novels ("Sundiver," "Startide Rising" and "The Uplift War") posits a huge galactic civilization (actually multi-galactic, spanning five galaxies) where a process like this called "uplift" is a major part of the structure of their society. Advanced alien species who come across another species likely to make good candidates for "uplift" into sentience have to petition to get the right to uplift these beings and then set about what is usually a millennia-long process to genetically engineer and breed them into a new intelligent race. These uplifted species are, by law, subservient to the patron species who uplifted them for a time (usually thousands, if not hundreds of thousands of years) before they are freed from their servitude and can themselves become patrons of other species. Patron species tend to favor client species that are similar to them, so often their clients look a fair bit like them. In the series, humans are considered something of an anomaly because it seems that we somehow managed to uplift ourselves into sentience, something that has almost never happened in the whole multi-billion year history of the five galaxies. Some aliens believe that we were partially uplifted into sentience secretly and illegally by some patron who then abandoned us before the job was done and we evolved the last little bit naturally. It's a fun series of books and offers some interesting ideas of how this sort of thing could happen.

2. The aliens we see are not the true forms of these beings. Some have suggested that Grays might really be artificial beings, if so perhaps they were designed specifically to interact with us and thus look more like us so as not to utterly terrify us. Perhaps the real beings who control them look very different, maybe they look like giant, disgusting slugs or something.

3. Why Nordic types? My own theory, assuming they even exist, of course, is that another alien species came down to Earth a long time ago (many thousands if not tens of thousands of years ago), probably happened to land in the region were Nordic humans lived, abducted some of them and then bred a colony of humans elsewhere. These would be humans, descended from humans on Earth, but who grew up on another world, somewhere and who were tutored by aliens. Due to alien genetic manipulation and breeding they are probably now generally larger, stronger and smarter than the average human on Earth, which is how they are often described in sightings.

4. The humanoid form is just very good for making sentient, civilization-building species. We see in nature in our own world how different species can fill the same ecological niches in different regions and tend to develop the same features. A humanoid reptilian dinosaur descendant was probably a very likely outcome, but there just never were the proper environmental pressures to create one. They might have had a different limb arrangement, more theropod-like legs, maybe, but still two arms with dextrous hands, two legs, a big brained head on top, etc., but essentially they could have evolved that way. Being bipedal is a very good design for intelligence, and civilization building. It frees up the hands for making things, is a less energy consuming way of getting around than four or six legs, thus freeing up the body energy output to be devoted to an expanding, more intelligent brain. So, it seems our design is a happy accident that can help lead to big-brainedness and that same design could well be repeated on other worlds. It may well be the most efficient design nature can produce that leads to sentience, but we'll never know that unless and until we start meeting other beings who look, more or less, like us.