"Most car accidents are frontal"
They aren't. They might be for one of the cars, but how often do you think two cars have a head-on collision? Think about the scenarios: pulling out of a junction, failure to stop in a line of fast traffic, spin/slide on slippery road: one car will take an offset, the other a side or rear impact. For both cars to take a frontal impact would involve, what? Overtaking accidents in which neither car swerved?
The problem is that most car accident tests are frontal (perpendicular to a concrete block), which is why the FIA has been funding NCAP to investigate offset accidents and side impacts.
Frontal impacts, when they happen, involve more energy and there's (usually) the mass of the engine and (almost always) the steering column and pedal box to worry about, which is why the driver's in the most dangerous position. And also, of course, there's always a driver in the car, whereas the other seats aren't occupied 100% of the time.
But the reason why there's endless arguments about this sort of stuff is that cars are getting better, long-term studies involve cars you aren't driving anymore and everyone's got an anecdote. For example, the middle seat is safer than the rear side seats if all three rear seatbelts are equivalent. They rarely are.