Because, like in so much else, we trade off safety for convenience.
Why fly at all, if you want to be inconvenienced, but safer? Okay you couldn't get where you're going, but it's (almost) impossible to be involved a plane crash if you aren't on a plane.
The real answer is that statistically, very few people are ever involved in a plane crash. Of those, statistically very few die.
As stated in my answer here, if you got on a random plane right now, and then at every airport immediately disembarked and got onto another about to leave, it would take you something between 20 and 100 million flights before you died.
So you have a 0.00000001 to 0.00000005 chance of ever dying in a plane crash. That's so close to negligible that it isn't a factor. And if you survive a plane crash, are you really likely to be grateful to your airline for putting you in the nice "ever so slightly safer" seats in the back?
The statistics of the "safer" seats is based purely on accidents which have a proportion of fatalities: it excludes accidents where everyone or nobody dies, and ignores injuries.
In short, you're looking at a statistically insignificant statistic about which seat is "safest" on a plane that will almost certainly not crash anyway, and even if it did, everyone would be probably be fine (85% of accidents have no fatalities).
Compare that negligible difference in safety, with the 4.5 million passengers per day, of which approximately half a million (10%) or so are likely business class.... and you're looking at massively inconveniencing 182 million of your best paying, most impatient passengers every year, in exchange for making them ever-so-slightly less likely to die in an accident they probably wouldn't be involved in in the first place.
If I'm in a plane crash, I'm probably either dead or not... my concern is not whether I'm going to use my frequent flyer miles, nor where I'm sat.