The reason is simple.. All these engines are sensitive to back pressure or the exhaust.

With all the differnet planes and differnet engines it would require tunning each 6 to 36 inch long
sealed exhaust for that engine....

I tell you this.... It would make some planes much faster because the exhaust velocity would
be around twice as fast.. ( On a sleek plane)... But because most plane can get to 200 mph
it is something that is not required and too much trouble..

Eddie Weeks