If you where an alien and you looked and saw how humans treat each other and other inhabitants on earth would you seriously want to come and visit? Thats even assuming they have the technology needed to get here. Chances are there are hundreds of alien beings living in different univereses some maybe more advanced than ours some less and some maybe just comparable to animals but it may be impossible to ever reach us even for a truly advanced race.

But bottom line is if they do have the tech then coming here would cause massive problems for us and theres a good chance people would try and kill them.