Or because no one has found a way around that pesky speed-of-light barrier, and the vast distances simply make inter-species communication, let alone travel, utterly impractical. This has always seemed, at least to me, the least romantic but most pragmatic answer to the question of why we don't meet aliens, or even hear from them.
I can't buy that, either. Intelligent machines must be possible -- after all, we're just meat machines, and unless there's some divine entity handing out souls there's nothing particularly special about us naturally-evolved organisms that couldn't be duplicated in an artificial organism. So it should be possible to purpose-build intelligent machines and send them out as interstellar probes. Make it so the intelligence can hibernate for the journey by powering down.
Now, let's say the probe is only moving about the same speed as Voyager, 17 km/s. We know that's easily achievable. At that rate it'll take about 17,000 years to travel one light-year. So let's say our robot probe travels 100 light-years to a nearby star (1.7 million years travel time) and sets up shop. After another 300,000 years it's ready to launch two more probes. Each of them goes 100 ly and repeats. At this rate it only takes 2 billion years to span the galaxy, and we end up with something like 10^300 (2^1000) probes. Maybe we ought to build in a limiter that stops reproduction when a probe hits an already colonized system...
Mind you, that's with some really pessimistic numbers. And it doesn't even need machine intelligence, I just think a machine has a better chance of functioning after a couple million years of travel than a hibernating meat popsicle does.