That would reduce the usefulness of the road for pedestrians to zero
On the contrary, they could cross anywhere without needing to walk to the zebra crossing! That would increase the road’s usefulness for them.
But pedestrians can do that already, so your proposed change would not change this; thus there could be no increase.
Now it is illegal in some places and not recommended in others → social & cívic pressure against. Plus the increase in usefulness for the cars.
On the contrary, they could cross anywhere without needing to walk to the zebra crossing! That would increase the road’s usefulness for them.
But pedestrians can do that already, so your proposed change would not change this; thus there could be no increase.
Now it is illegal in some places and not recommended in others → social & cívic pressure against. Plus the increase in usefulness for the cars.