New cars have engine designs that used to be found only in Formula 1 cars to meet pollution and mileage requirements. Plus the 8 - 9 speed automatics.
This has to adding somewhere between $5000 - $10000 in costs for the engines and transmissions to new cars, if not more.
It also makes them impossible to repair when they break down: they'll have to be re-manufactured.
The sad thing is, minus the crash test mandates, you could easily get the mileage because the vehicles would be much lighter. I remember my first Ranger - a 1982 1/2 short bed. It weighed 3000 pounds, had a 2.3 liter 4 cylinder engine and a 4 speed stick (they hadn't developed the 5 speed yet). I regularly got 30 mpg on the highway at 60 mph or so.
Fast forward to the mid 90s and the Rangers had gained 1000 pounds. And the mileage fell proportionately. Recreate that 3000 pound truck and you'd have a mileage king with today's engines. But you would undoubtedly not meet the latest crash tests.