Capital has historically been a big factor in that equation as well. A factory worker couldn't very well take his machine around to different factories, whereas a mechanic probably has his own set of tools. Those of us in the computer field are fortunate that, at this point in time, capital requirements are very, very low. In other fields, though, large amounts of money are needed to be able to get off the ground. Who knows what the future will bring... less capital needs or more?
Incidentally, as much as I would have liked to see it, I couldn't take watching a bouncy image of some guy's ear next to a tiny Paul Graham. It would be great if they provided video of the whole thing for those of us not able to be there.