All dead bodies that were found in cars that did not reanimate probably suffered head trauma, preventing reanimation. As for starving to death, there may not have been enough time for any walker to do so. And anyway, only active walkers would attack, so the sampling of walker demographics is biased by the dramatic needs of the show.
I always thought those people weren't infected when they died and once dead . . . couldn't be infected. This was in the early stages of the Zombie outbreak before a form of the infection somehow became airborne (or whatever) and infected the living. Yet apparently this strain doesn't turn the living but only reanimates the dead.
Although even describing that idea seems absurd on so many levels . . . but . . . hey . . . its an entertainment show not Bill Nye the Science Guy.