Mostly, they don't. Boarding priority is governed by a few factors, most prominently giving the frequent fliers and first-class ticket holders the feeling like they're getting their money's worth. Coach is boarded back to front because a) coach is (semi-intentionally) treated like luggage which moves itself and b) boarding back-to-front is faster that front-to-back, in that it has less "person stowing their bag blocks the aisle" moments. Saving 10 minutes on boarding per plane per trip is worth, literally, hundreds of millions of dollars.
Remember, if boarding is screwed up and slow (and studies have suggest that it's significantly faster to board window, middle, aisle, all things being equal), then early boarding is a privilege/perk they can sell.