You are confusing 'bare land' with 'land with planning permission for a dwelling'. The latter is very valuable. Given planning permission for N dwellings, it is N times as valuable .. even if it isn't N times the area.
The cost of =building= a house =is= going up, but much more slowly than the cost of the finished 'house on land' .. =building= costs are going up roughly in line with earnings (plus a bit, usually, as minimum standards increase, and taxes and red tape go up too).