Following Matt Wilson 's musings on Google+ on Star Frontiers vs Traveller... and this is almost entirely tangential.
See, Matt says that SF had lots of unexplored space, whereas the Traveller 3rd Imperium was well settled for millennia, crowded and no longer frontier.
It's only in prepping for my (much delayed) campaign for my son that I've realised that this just ain't so, and that the Imperial Scout service will never go out of business.
And it comes from looking at the system generation... system (Wait, what?)
Okay, the basic PLANET generation system generates the main world in a solar system . Since it's skewed to produce systems somewhat similar to ours, they've got a loooot of surface area (mainworlds tend to be averaged around earth sized), somewhat non-hostile atmospheres, and some reasonable amount of surface water.
And the average population of these worlds?
500,000. About that of Akron, Ohio. Or Sheffield.*
On a whole planet.
And the other planetary bodies in the system are likely inhabited, but even more sparsely.
So population densities are very, very low.
So that's the mechanical basis for there being an awful lot of "uninhabited" planetary areas, especially considering that in a technological (but not post-singularity) society, urban dwelling is going to be the norm. So even the few people on each planet are likely to be concentrated together.
So what's the in game reason for this?
Cheap, long established space travel.
In economic terms, once you've gone 500 miles, you may as well go two parsecs. The marginal cost is negligible above a certain tech level**
So human occupation of space is very wide, but very, very shallow. Even after 3000 years of human occupation, a planet could be barely explored outside the "civilized" areas.
I mean, we're still finding "lost" tribes, unknown species, etc. on Earth after, what, 5,000 years of civilization, and we've got nowhere else to go.
It's a very widely distributed civilization with a lot of nearby frontier. Not only could anything be out there from pre-human occupation, but any number of humans could have gone into the black and got lost... and never even needed to go into jumpspace to enter "the black".
Then there's the "uncharted" areas in the underworld of those tight concentrations of the cities, but we're already in Joseph Conrad territory enough here.
*/I was going to say "Liverpool", but then found that if you include the Wirral, over the water, this would be closer to 900,000, but that's still within "population; 5" for Traveller./
**And to an extent, at lower tech levels interstellar trade is CHEAPER than planetary trade: pre fusion plants, you've at the mercy of local fuel costs....