I'm not an urbanist, but my feeling is that the size of the lots and the winding paths of suburbs make the paths from any house to any shop a lot longer overall, so you end up with the need to drive to a store, which leads to parking lots and being in a main road.

In a tight grid (like where I grew in Chile), a small store can survive off the foot traffic of a few square kilometer of houses, because they're more connected and just more fit in the space.