The human body is not perfectly elastic, as it is made out of meat. When the bike goes up and down, it makes the meat jiggle or flap which dissipates energy in the flesh as heat. So taking the hit at the tire which is much more elastic instead of in the heavily damped rider is more efficient.

Take a look a slow motion video of riders on a rough road, say some Paris-Roubaix footage and you can see this.