Probably for the same reason we haven’t contacted them!
Suppose we discovered a planet with aliens. How would we contact them? We wouldn’t be able to travel there easily. It would likely be many light-years away. To travel that distance would take hundreds of thousands of years. Sending a signal there announcing our presence would take many years because of the speed of light.
So I’m guessing for the same reasons we would have, aliens would be pondering about how to contact us!