When you're a student or whatever, and you can't afford a car, or a plane fare, or even a train fare, all you can do is hope that someone will stop and pick you up.
At the moment we can't afford to go to other planets. We don't have the ships to take us there. There may be other people out there (I don't have any opinions about Life Out There, I just don't know) but it's nice to think that one could, even here and now, be whisked away just by hitchhiking.
Statement of 1984, as quoted in Don't Panic: The Official Hitchhikers Guide to the Galaxy Companion (1988) by Neil Gaiman, p. 2