It's because suburbs are subsidized in the current system. Roads, infrastructure, and other upkeep costs are paid by the entire city but since the denser region

It's because suburbs are subsidized in the current system. Roads, infrastructure, and other upkeep costs are paid by the entire city but since the denser regions are more productive/profitable, they end up paying for the services that suburban residents are using. If suburban residents had to pay for the real cost of their housing, then I think it would be a lot less appealing. Building suburbs is terrible city planning and it's sad that cities are continuing to endorse them.