“especially given that a net new refinery hasn't been built in the US in 50 years.”

Existing large refineries have done some massive expansion projects in the last couple of decades, adding the equivalent of a several new refineries. It is often easier to do this than build a new grass roots refinery.

Example projects:

MPC Garyville +180 MBD (2009) Motiva Port Arthur +325 MBD (2012) XOM Beaumont +250 MBD (2023)