Why haven't aliens contacted us?

Some people believe that the lack of outside contact is evidence that complex life does not exist in the universe.  As the idea goes, we know that simple life is easy to create (from our own experience).  Yet, complex life defined as a civilization at or beyond our level apparently does not exist.  If they did, presumably there would be a lot of them, and we would find evidence of them in the frequencies of the cosmos.  So some people believe life is easy to start but hard / impossible to develop beyond a stage where we are, the conclusion being something snuffs out the civilization when it is about our age.  Maybe nuclear weapons are to blame.

There is a tribe deep in the Amazon jungle in Brazil that has never had contact with humans from the outside.  The Brazilian government is actively protecting them from outsiders who are trying to contact them, in fact. If anyone is curious why we haven't been contacted - this is my theory why:  we are the equivalent to the uncontacted Brazilian tribe.  There probably are millions of intelligent civilizations, and maybe even the supreme among them are protecting our planet from outside signals from others who would interfere.  In their mind, we should make our own way.  If we succeed then we can join the league of universal civilizations.  If we destroy ourselves in the interim, then we weren't a worthy species.

I think Bill Watterson's theory is good too: