The idea of tolling roads may work well in areas with high traffic volumes. However, what about rural areas that may only see a couple thousand vehicles a day? Take a state like Montana for example. We have a few urban areas and a huge amount of rural roads. Even parts of the interstate have an annual average daily traffic (AADT) of less than 4,000 vehicles a day (I-15 near the Montana/Idaho border). [1] If these were toll roads, the toll would either have to be too expensive or it would never actually pay for itself.
Well, look at the original purpose for the Interstate. One of them was for the military to be able to move freely should something happen. The actual name is "Dwight D. Eisenhower National System of Interstate and Defense Highways"
[1] https://www.mdt.mt.gov/publications/datastats/traffic_maps.s...