That's because the Jags cost an arm and a leg...trust me, the people that can afford them are not worried about fuel efficiency.
They made big cars too...the AC Cobra housed the biggest stock engine ever made for a car...and while the egine was American..the Cobra was originally a European Car.
The major difference is that Europeans tended to place more importance on handling while Americans valued straight away speed...and most of the mid cost American muscle cars were fast enough to keep up with the $$$$$$$$ fast cars in Europe. Now fuel efficiency is a concern....but part of that is self inflicted...they tax the crap out of gas over there, but they pay the same amount for their Oil that we do.