IIRC because more of the highway cost is subsidized by the taxpayer, not necessarily because the cost is actually lower...

In the US, especially outside of the northeast, most interstate highways are not tolled, so users of the road don't pay any more for its maintenance than non-users do. But someone is still paying for it. Taxpayers who don't use the roads are effectively subsidizing those who do.

By contrast, railways usually own their rails and are obliged to maintain them themselves, and thus have to fold those costs into the price.

This is really unfair.