It's a problem because it's a positive feedback loop. More highways = more traffic, which leads people to want to build more highways to supposedly ease congestion, and this continues ad infinitum. Our cities grow more spread out, less walkable, and less sustainable. Overall it makes our cities worse both for the people who live there and the environment.

If we didn't subsidise highways so much, demand for them would be reduced, and it would make more sense to build denser housing and provide public transportation.