Several people have answered this from the angle of why the cost is large, but missed the other (necessary) angle - the benefits are also massive and well understood. Plenty of potential projects have huge costs but they don't exist because the benefits aren't big enough, highways exhibit a sort of survivor bias in the mega project world.

Highway systems create massive economic benefits that are relatively simple to calculate, just in the raw # hours saved which massively increases productivity. This can be well captured by the state through taxes or (less frequently) by private industry through tolls - i.e. the people that direct their construction.

Other mega projects have much less certain benefits, or the benefits are harder to capture by those with the ability to allocate resources because incentives aren't sufficiently aligned.