I think about it from an aerodynamics perspective. Think about the effect the plane would have on the air just above the ground. You have a plane coming down that would pancake that air, which would actually push the plane upward (if the pilot were truly trying to keep flat as opposed to diving). It creates an oscillating effect on the plane and would make it very hard to control.

You can even replicate this with your hand if you have a source of very fast moving air like a compressor hose pointed along a wall. If you semi-quickly approach your hand to the wall, but don't try to actually hit the wall, like the pilot not trying to hit the ground, you'll actually feel your hand press against the air and then your hand will lift off the wall for a moment.