Star Trek

So why in Star Trek so all the aliens look like us - just a human with different ears or a forehead?

Well SF is a cultural thing. We want to believe the Enterprise will find that there is life out there - that we are not alone. It's a comfort thing. The possibility that we're alone is frightening. And we want to find M class planets, like ours, and life "as we know it", albeit with a few variations.

It's the same as when the first sailors, Magellan, Columbus, etc, sailed across the oceans and found new "worlds" inhabited by humans like us, but with different coloured skin, different hair, different eyes and noses. We want to believe the new worlds we find in space will be populated with people, just a bit different. That on all these new worlds, God has still created "man" in His image.