We still do for piston aircraft, thanks to intense lobbying by the aviation piston engine industry.

There's even a 100LL alternative that has sailed through most tests the FAA requires but the FAA has been stonewalling them for something like a decade. The FAA is full of paper-pushing corrupt beaurocrats who are firmly in the pocket of industry, as demonstrated by the thousands of victims of Boeing crashes from the idiocy of the MAX program (wherein Boeing did not want to spend the money to redesign an aircraft for bigger passenger and cargo loads, so they just stretched the plane, which put the Cg out of whack, which meant they needed to have a computer help fly the plane...and then skimped on redundancy.)