Or, its too much work for them to fund SETI research on their planet... and they find that they can spread through the galaxy faster, if they just wait for a civilization to a) announce their presence; b) nuke themselves to the stone ages; c) and leave lots of space for the aliens' 100-year starships to drop off a few million colonist to the 'intelligent' species that just nuked themselves. Slow and steady wins the race.