This whole "ET" nonsense is completely based upon assumptions - a thought experiment, if you will. If, what many scientists say about life is true: that life is inevitable under certain conditions, especially those types of conditions that existed at the time of the first life on Earth, then there has to be literally 100's if not 1000's of inhabited worlds, and of those planets there has to be at least a few species that are sentient, rational, and intelligent. If you look at man, it took us 3-4 billion years to get here, so it's not unreasonable to assume a similar time period for the development of a "similar" organic based life form. To top this off there has been plenty of time since the beginning of the universe for this to have happened - more than once.
If they're there we should see 'em, and I guess some folks think we do.