Answer: Because nobody's built one that's better than buying a car and an airplane separately.
We also got into airships and personal flight backpacks. The rest of the experts didn't like me sticking to the actual question, so David Spitzley supplied examples of how a gamemaster could modify his world to make a different technology come out on top. I appended this to my answer while muttering "scope creep is bad" into my sleeve. For any gamers with $20 to spare, the full article is at:
. . . and Pyramid covers lots of other fun stuff, so give it a try.
So this solves how I'm getting GURPS 4th Ed. volume one. Now I'll have to see what happens with "The Hills Are Alive With the Sound of Panic."