In my opinion it is inertia and familiarity.. you use your "standard" transportation mode. The default transportation mode in all but a few core urban centers in the United States is a motor vehicle. Once a motor vehicle is secured and available, it will be used for any transportation purpose that doesn't involve an obvious excessive marginal cost or delay.
It works both ways. My default transportation is the bicycle, and for me, driving the motor vehicle is a hassle.