If 'They' were smart enough to get here why hide.
Maybe they don't like sunlight?
I cannot see any valid reason why the universe wouldn't be teaming with life. Just because we aren't advanced enough to find it, doesn't mean it doesnt exist...or they can't find us. Why they would want to...who knows. But...they might. Maybe for the same reason we want to know if life exists elsewhere? Or, maybe they have destroyed their planet the way humans are destroying ours...or maybe an asteroid destroyed their planet...or their sun burn out...or...there may be a million reasons why aliens might want to come here. It's a pretty cool planet.
Recent discoveries are starting to show more and more that it is possible that life exists elsewhere. The universe is made from the building blocks of life. And there are billions of galaxies with billions of stars with planets that are the habitabale zones in their solar systems. Even the moons around Saturn are proving to be very interesting. Now they realize that volcanic activity, and places like the antarctic can also support life. Not life as in human life...but life as in microbial. It really is amazing. Just imagine how much more we will know in 100 years time. I wish I could be here to see the knowledge then. At the rate we are learning...it would blow our socks off.