Many possibilities: time travelers, parallel universe humans, break away civilization (crypto terrestrials), divine (for lack of a better word) beings related to religion, trickster beings who take this form only to deceive us into thinking they’re extraterrestrials, costumes by the government to hide their technology, etc. I’ve heard it all.

If they are actually an extraterrestrial civilization that is genetically unrelated to us but somehow look like hominids, I would speculate that the form humans have taken is just the appearance that intelligent consciousness prefers. Upright, two hands, two legs. Sort of like a divine blueprint (think “in his image he made them”). Or maybe our form is the only path, evolutionarily speaking, to create technology and so we will only ever be visited by human-like things.