I have heard that when we look it to space, we are looking in the past, so even if we do life signs of life, it might no longer be there?, so we earth must also be living in the past?, if you know what I mean
Also if they were 100 light years away, by the time we received their message and they got our reply 200 years would have passed.
It's also possible that more advanced civilisation have long ago abandoned radio waves for communication and are looking at our transmission like we might look at smoke signals