This always used to make me ponder too.
As a kid I watched Doctor Who, and I sometimes wondered if Krypton was a little like Gallifrey -- on Gallifrey you had a race of people so technically advanced that they could travel in time, yet the only examples of Gallifreyans outside of their home planet tended to be rogues, renegades, or oddballs. The sophistication of the Time Lord race meant that they became aloof, distant, and prone to introspection: they simply retreated to their home planet, almost like gods to the top of Mt Olympus, and had little reason to interact with the other 'lesser' planets elsewhere.
The other possibility is that there were religious or cultural reasons why Kryptonians didn't go off planet much. Almost like a cosmic version of the Amish, they chose to isolate themselves for cultural reasons. They
could visit other planets, but they preferred not to mix.
And then I think of how many Americans don't even own a passport, let alone have ever strayed outside of US borders, and I begin to wonder that maybe it isn't so mad after all that a technologically advanced and highly sophisticated culture wouldn't have too many people off planet.
R5