I mean even on our own planet, there’s such a wide variety of life and most of it looks widely different from humans aside from fellow primates. Why should they look anything like people, and not like cephalopods, or insectoids, or more likely something vastly different in design. They would have an entirely different evolutionary lineage - and who’s to say they’d even have a genetic system like our own? Like you said, we’re the only sample we have. You can’t make assumptions about a galactic or universal population based on one planetary sample size

Of course, this could be entirely wrong, and maybe extraterrestrial life is more similar to us than we’d expect - but even then, there’s no reason to expect any life forms we encounter to look like us any more than we should expect any other species to. Sure, a few may, and there may be similarities, but to assume they’re likely to be at all humanoid just feels like heavy anthropocentric bias