9. They're Not Developed Enough
The universe has existed as it is for 14 billion years, while Earth took 10 billion years, and of that history, humans did not emerge until recently in the scheme. Early humans went through many stages, from hunter-gatherer to agrarian, leading to the rise of cities and nations. History can be traced back thousands of years but after that, things start to get fuzzy.
As previously mentioned, it was not until the 20th century that humans were at a point technologically that contacting aliens was even feasible. It took the species 99% of its history to develop a sufficiently sophisticated understanding of the world, all while prospering in other fields.
If you have an idea of alien life in your head, it's most likely a depiction of a super-advanced and technologically superior race. This fuels the narrative of cinema and television, but it's equally likely that while humans send complex messages out into the reaches of space, they have yet to master fire.