Why do aliens always have to look and have similar biology to us?

Exactly! To date, no life has been found on any other planet or meteor. We would really like to find life elsewhere, and the most likely candidates within reach would be the moons of Jupiter and Saturn.

Beyond that you have to go interstellar. Since it would take hundreds or even thousands of years to reach even the nearest stars at the fastest speed we’ve ever achieved in space, interstellar travel is unlikely now or in the future.

Put away Star Trek and Star Wars. Aliens are not visiting us and we are not going to visit them, whatever they may look like.

If 50 years into the future, an alien civilization comes to Earth to invade our planet and only you know about it, what will you do?

Sneeze. That would probably kill them all off since they would not have developed evolutionary immunity to our germs. George Orwell was quite right.