Efficiency matters. If allocating 100 million currency units to widening a highway from 5 lanes to 6 lanes improves people/hour capacity and quality of life less than other solutions, the other solutions should be chosen. In addition to diminishing returns on the way we’ve always done it, there’s a hard limit of about 2000 cars/hour/lane.

Chocolate is not free at the point of use but roads usually are. Without price signals or other limits people over-consume resulting in a shortage and people paying in time.