Maybe because nobody is going to invest megabucks longterm to create capacity only needed in the short term? (and may not even be needed)
In the chart linked here [0], section "Gasoline stocks (million barrels) and days of supply" each Petroleum Administration for Defense District has had flat supplies, indicating no especial need for additional refining capacity. In "US Regular All Formulations Gas Price/Consumer Price Index for All Urban Consumers: All Items in U.S. City Average" [1] the CPI adjusted cost of gasoline is somewhat high but still below where it was 2011-2015.