Because we don't know how life was created here, we also don't know much about our own solar system, and amazingly enough we know even less about systems around other stars. Imagine walking into a giant warehouse full of refrigerators, going into the break room, looking in the fridge and seeing a bunch of food, then thinking to yourself, "with all those other fridges out in the warehouse, there must be all kinds of food here".
It is fine to believe that life exists beyond Earth, it's fine to believe that it exists because there is so much stuff in our universe, but that argument isn't irrefutable.