My theory is that we haven't met aliens yet because the aliens have been to earth sereptitiously, have taken some DNA samples, and have decided there's nothing to be gained from making contact with us. They've told everyone else in the universe that the planet is hostile and not particularly technologically developed. They have also noted that it is not a threat just as long as they don't get advanced space travel technology. The best way to keep them from getting advanced space travel technology is to not communicate with them and thus leave them to their own devices. Additionally, based on the alien's models of planetary development, they have come to the conclusion that our civilization will not develop decent space travel before it collapses due to natural resource depletion.

The universe is a big place. There are probably trillions of habitable planets. We're just not that special. Maybe one day, if we somehow become "worth it", they'll stop by.