On the subject of turbo fan jet engines, the main reason why they are not practical for cars is that they are rather low torque at lower rpms. Think the old slushomatics of early Chryslers. Sure, if you rev one up until it howls, then you have to deal with the howling. And the transmission/reduction gear would take a beating.
Modern engine/trans combinations are more fuel efficient while still offering decent performance by having far more gear ratios than older three speeds. Nine and ten speed trans are becoming common. This allows the car engine to stay in its sweet spot, keeping revs and fuel consumption down while still offering acceleration when needed. So now modern automatics can be more fuel efficient than manual trans, although a stick shift is always more fun to drive IMHO.
As far as limited space between bumper and grille/trunk lid, etc. I believe Chrysler more or less led the pack with that approach in the late 1990's. They had adopted advanced computer design technology, along with AMC's platform approach, and were able to bring new somewhat trend setting models to market a year or two before the competition.
Aerodynamics and styling are one reason. But another reason is that government regulations do indeed affect bumper design. I think it was back in the 70's or 80's that the feds started requiring car mfg's to design the cars so that they could withstand low speed collisions (like maybe less than 5 mph) without suffering a major repair expense. The way modern cars seem to handle this is to use a Styrofoam form for the jbumper, with a very thin vacuformed shell over that. On top of that, they make the shell relatively inexpensive to replace. But it does impose some design constraints and has been a big gripe for fans of European cars who find they can't buy them here in the USA because the foreign car company doesn't want to go to the bother and expense of ruining their sporty design to meet American regulations. It's probably less of an issue today, what with the foam filled bumpers, but I believe it is still a gripe for some car nuts.