Prius's are city cars, city driving has widely varying loads which are inherently awful for combustion engine efficiencies. The hybrid drivetrain allows the engine to stay within its optimal efficiency range which has a huge impact on mileage.
Compare this too semis which spend most of their life on the highway with drivetrains optimized almost exclusively for constant highway speeds. There is far less gains to be made converting to a hybrid system.
A Prius without a hybrid drivetrain and with an engine optimized for 25 mph driven constantly at 25 mph would actually exceed the standard Prius's mileage since it doesn't have to carry batteries, generators or electric motors.
Compare this too semis which spend most of their life on the highway with drivetrains optimized almost exclusively for constant highway speeds. There is far less gains to be made converting to a hybrid system.
A Prius without a hybrid drivetrain and with an engine optimized for 25 mph driven constantly at 25 mph would actually exceed the standard Prius's mileage since it doesn't have to carry batteries, generators or electric motors.