Perhaps without FTL drive, it takes what explorers there might be a long time between (expensive!) visits, so sightings are rare. Without FTL, and without some kind of cold-sleep, a journey to the nearest stars pretty much takes a lifetime. A few would go, but not many.
It's too late (bedtime) for me to slap "Drake's equation" type numbers on this, but hopefully you get the idea. Maybe we've not seen anybody because they only visit any given planet every million years or so, and "they" equals 2 or 3 species local enough to even bother with that paltry schedule.
I'm not saying there is anybody out there, just that lacking pure freaking magic technology, it's a hell of a trip to make if they were. OTOH, if there is an outpost 100 light-years away, expect a visit in another 100 to 1000 years???
Any species that didn't have intellectual curiosity about things that aren't obvious useful would probably never make it off its home planet. A lot of the math and science that underlies stuff we use every day didn't seem useful when it was developed - see, for example, the theory of computation, worked out when there were no computers, and without which there probably still wouldn't be any computers.
It may be electromagnetic leakage or Dyson spheres or any of a number of other signs of artifice.
If the "we're like ants/barnacles to them" theory is true, then they wouldn't bother intentionally hiding from us, because we're no threat to them. If they do it unintentionally, that would still be interesting: for example, it would probably mean that civilizations do not in fact aggressively colonize the galaxy.
Let's imagine there are far more planets like ours that have life - let's throw intelligence out, because that is defined by some relative standard - which has not yet reached that capability. Why would we be interesting in the scope of these far more advanced extraterrestrial societies?
Perhaps they don't reach out because it would be as fruitless as us trying to communicate with ants. Perhaps they don't care to study us because we seem about as interesting as primordial soup (ie. a nuclear holocaust might be a trait of that). Perhaps intelligences like ours are well understood, well classified in the genus of the universe, and we are about as ordinary as a barnacle on the hull of a tug boat.