Because we would detect them otherwise.
Through what means though? We are not by any metric an advanced civilization, we would not even rank type 1 on the Kardashev scale.
It would take a huge technological effort to totally hide yourself to the point that not a single detectable signal escaped. Anyone looking in at earth would see the equivalent of the New Year fireworks on Sydney Harbor.
Assuming of course that they emit signals that we could detect or even recognize from across the galaxy.
So you are not only assuming there are aliens but that they are doing something impossible like travelling faster than the speed of light (but are then still to incompetent to continue to hide themselves once here)? I think we assume things we see in Science Fiction film and books are real or even possible.
I'm not assuming anything mate that was the point of my post. Also there are many scientists who think there are ways of travelling faster than light that are theoretically possible.
https://www.popularmechanics.com/science/a35718463/scientists-say-physical-warp-drive-is-possible/
If there potentially was another intelligent civilization out there, capable of traversing the galaxy. Then they are likely tens of thousands if not millions of years older than our civilization, in that scenario who knows what scientific breakthroughs they have made.
There is do much amateur and professional observation and monitoring that it beggars belief that we never ever spot anything.
You assume.
A bit like looking for God. You can't prove a negative but the utter lack of even one half convincing evidence is highly suggestive. I wish it weren't so as I'd love for us to encounter alien life.
Is anyone looking for God though?
I don't think we will encounter alien life in our lifespans. I have no idea if there are intelligent aliens species out there. But there is almost certainly life on other planets that we will encounter in the future if we colonize the galaxy.