Or more likely, development costs.
A new engine can cost millions to design, get through the rigorous testing and approvals procedure and then market,.
If you are putting it in a car or any other vehicle, more than likely you'll sell millions of them and get a rapid return on you investment.
If you put it in an aeroplane, you might, if you are very lucky, sell a few hundred eac year, and you'll be waiting a very long time to get your money back.
I mentioned this to a freind who was formerly a Lotus bodywork and engine design engineer and is now working on a brand new concept car with a new small but specialist firm.
He was suprised and intrigued that none of the aircraft manufacturers appear have attempted to work with the automotive industry (or vice verca) to reach a compromise, and intends to pass the idea on for further thought.