Aliens in Science fiction movies

Watching star wars episode VI, you really have to think ... why do all these sci-fi writers insist that alien species no matter from where would have structures extremely similar to those of humans? Why is it necessary that they need eyes ? Why 2 eyes ?? Skin ? Mouth ? Why do they have to stand on two legs .. Why do they have to communicate by vibrating the air (like us) .... and have limbs ??

Why would they need 'mechanical' vehicles to move.. controlled by computers..

All it takes is some extra imagination to be able to think of and picture a different world .. a world - say - in which breathing doesn't happen wouldn't need a similar order of holes as us for god's sake (holes themselves are not a necessity, but let's assume)! the processes of breathing, feeding, talking moving through friction are in no way universal but rather dependant on very specific conditions found on earth...

They are nice movies though ... they make you think beyond ... 'may the force be with you' ...