Actually they do. I've replaced bearings in machinery, where the only fault has been excessive play. Otherwise they've run perfectly smoothly. These are bearings that have not been subjected to any shock. On a wheel bearing, I would assume that once any significant play develops, road shock could crack the ball or race surfaces, leading to rapid failure.
With larger bearings that may be true, but IME bearings up to 75mm diameter at least are hardened right through. Maybe the balls, rollers, and track surfaces are slightly harder than the rest, but the difference, if any is only slight. Mike.