[HankB99]

That does not seem so far fetched to me. So many of the purported alien sightings are beings with bilateral symmetry, two eyes, two arms, humanoid face and so on. The only way I could see that happening is visitors from a distant future.

Or, more likely, the alien "creators" have created them more or less in our image.

[tzs]

Another possibility is that the Galactic Federation has a rule that when they need to visit a planet that is not yet aware of aliens the crew must entirely consist of beings that have the same general form as the people of that planet.

That makes it harder for someone who sees them to convince others that it was aliens and not just a trick of bad lighting or someone with deformities or injuries that give them an unusual appearance.

It also makes it less likely for them to be mistaken for some other species on the planet. Suppose the Federation sent an expedition to ancient Earth that included crew from a species that looked a lot like our cats, and accidentally let Earth people get too good a look at them and their technology.

Those Earth people might think those are Earth cats, and conclude that Earth cats are a lot more powerful and advanced then they thought, and that they had better stop treating cats like animals lest the cats decided to wipe them out and start treating them as superior beings. Next thing you know that entire civilization is treating cats as magical being of great value or even worshiping them as gods.

[HankB99]

Having owned (or more properly cared for) cats, I've always been a little suspicious.

[shiroiushi]

It's quite possible that the humanoid lifeform is optimal to have a technological species that can travel between stars. An aquatic species would have huge difficulties just building technology and civilization, because of the habitat. A species without arms and thumbs would have a hard time manipulating its environment (just look at all the 4-legged animals now). A species with more than 4 limbs would likely either be too small (insects) to accomplish much, or would need too much energy (and probably evolve to lose the extra limbs over time).

There's good reasons to think that alien species might not look all that different from animals on this planet, simply due to physics. Animals here didn't simply spring to life in their current form; they evolved from single-celled organisms to best suit their environments.

[throwanem]

The word 'parochialism' positively vaults to mind. Not to mention your flagrantly unjustifiable opening assumption, given the number of bipedal technological species known to us to travel among the stars is currently zero.