Why do aliens have to be organic and then have head, hands, legs and others? Cant alien be something we never imagined b4?

the equation just a theory. We cannot calculate things we havent encounter before. Ya i already had the idea of aliens being something we never imagine b4 but the movies/alien encounter and others always suggest alien looks like humans, have head body leg and hands.

This could suggest that the alien encounters are all hoax or fake because they couldnt think up of an imagination for aliens beside something like humans