LP records are thicker in the middle. Some outer edges are thicker than others, but generally, not as thick as the centers (labels, etc.). So, stack 'em like pancakes and they will sag and not uniformly - they warp. The warp sends a very low, sub audio, frequency into your gear, drives it nuts, keeps changing the VTA (relationship between the stylus and the groove walls) and generally makes a mess. Besides, how are you going to pull out that favorite disk you havent played in a few years when there are 57 records piled on top of it? Stay vertical, friend.