I see two types - one is the one pass of rough mow at 5-6 feet, for maintenance reasons described by Anthony. Edges are always hard to maintain, and I suspect maintaining a fw/bunker edge is even harder. It might end up that the turf in front of the bunker is even rougher than rough in some cases. Add in that most courses are trying to take bunkers out of play for speed of play reasons, or at least have been. If you look back at the 50-60's era books on architecture (thinking Cornish-Graves and others) they recommended building fw bunkers up above fw grade, mostly for visibility, but some wrote that it was to keep shots from rolling in (shorter shots) while the longer players could fly one in.
Even then, at 5 feet, and typical 1-2" rough height, perhaps one big bounce still gets you in the bunker? Or a hard shot (which would have gone further off line) reaches it, but one just petering out as it leaves the fw (which would have been a closer miss) stays out? As Pat says, there is a certain element of fairness to that, in most folks mind.
Second, is the situation where the fairways have been narrowed tremendously for cost reasons, and the fw bunkers in general are getting 5-10 yards from the fw edge. That one always looks ridonkulous and renders the bunkers almost useless.