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.
The newest large refinery built in the US was completed in 1976 in Garyville, Louisiana. Since then, while some existing refineries have expanded, new refinery construction has faced significant barriers in environmental regulation, permitting, and local political opposition.
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.
https://www.eia.gov/petroleum/weekly/gasoline.php
https://fred.stlouisfed.org/graph/?g=p93w