In Star Trek, there's an in-universe explanation for why most aliens have a humanoid appearance. Is there an explanation for why it is also the case in the MCU and other versions of the Marvel universe?
It appears to be because the Celestials manipulated life throughout the universe to be in their form to protect their embryos. As shown in Eternals Vol 1 #1 they created three races on Earth in their image: the Deviant, the Human and the Eternal.
That shows they used apes as the main life form to create these species but they also did it on other planets where apes were likely not present and so they modelled them after their own image. This is shown in the alternate universe Earth-9997 and in Earth X we see the description.
In the MCU I'm pretty sure nothing about this has been said yet, or at least I can't find anything. Will update if I find any new information though I doubt there will be any.