This estimate was based on a projection of our technological state 10,000 years or more from now, but without super-intelligent AI or FTL travel. So human settlements are all over the galaxy, but they are fragile and disconnected from each other by hundreds and thousands of years of travel in slow time. One of the problems in this world is a civilization may have crumbled in the time it took to travel there and be in such a reduced state that its incapable of refitting your ship, leaving you marooned.
https://en.wikipedia.org/wiki/A_Deepness_in_the_Sky