[varon]

One possible explanation is that interstellar travel is just too costly.

Typical human response. Who is to say that if "life/civilizations" existed elsewhere that they would know of or have an economic or monetary system.
Even here on earth we have many examples of "civilizations" existing and functioning without money or other economic concepts.....bees, ants etc. These creatures exist and accomplish everything necessary to fulfill their lives without an ATM machine. How refreshing.

[discostu]

Cost doesn't have to be dollars. Cost as a concept originated around resources, not money. Energy has to come from something. Given our current methods of travel in space we'd have to pretty much empty ANWR to get anywhere with a ship large enough to carry enough people to make a colony (which, because of recessive gene traits in breeding populations would actually have to be a lot of people if they were to be stand alone and not repopulated from Earth in a few generations, and they'd have to be stand alone because it would take a couple hundred years to get anywhere). That's a lot of cost.