I think the problem is simply that most motorists do not have the experience to determine how much space a cyclist actually needs. And more than that, cyclists, as a whole, have not built up trust that we are not going to suddenly swerve out into the road or cut across traffic on a whim.

When I am out on the road, I spend a lot of my time directing traffic around me. Drivers appreciate this and often honk or wave to let me know it.