Regulation, education, inertia. All of those tell specialist things like "streets have to be at least this wide in residential areas". Zoning can make it impossible for the requirements of daily life to be in walkable distance.

There is also some scale issues. A walkable city is great. A walkable neighborhood might not be, not if it is too small and everyone has to drive to work, to the grocer and to the doctor. Public transport makes only sense if it can move enough people. All drivers (so everyone) will be uncomfortable with the walkable, but disconnected small neighborhood.

There has been some rethinking in the last decade or so and cities are trying to become more walkable, more livable and less car focused with increasing frequency.