I had an idea on why the kerbals living/working at the KSC appear to be the only living creatures on Kerbin, as well as why they are so totally focused on space exploration.
The Monoliths:
My theory is that the Kerbol system is in the same universe as the aliens (who created the Monoliths) in 2001: a space odyssey. There are numerous monoliths dotted around the Kerbol system, with no apparent explanation for their presence.
Disappointed by the progress of the human race (who were more focused on fighting then exploring) the aliens sent Monoliths to another star, in an attempt to create and cultivate another space faring species. They created kerbals from the primordial soup, making sure there were only a few hundred to prevent significant tribal wars/conflicts. The aliens left the monoliths on Kerbin and around Kerbol to monitor the kerbals.
The reason the kerbals are so obsessed with going into space is because the aliens/monoliths have programmed this into them since they were born. The aliens chose the Kerbol system for the life because the lower gravity and smaller celestial bodies made it easier for the kerbals to build rockets and explore.