Why do high-speed water impacts / ditching usually result in structural loss of the airframe? Plywood sheets can penetrate tree trunks unscathed - when pushed by hurricane or tornado winds.  Why wouldn't an airplane fuselage, wings and tail behave the same way impacting ocean water at a steep angle at 300-500 mph?  That is, penetrate and remain intact. I have seen a lot of supposition written about this which seems based on common sense, but common sense might be misleading.  Has  the outcome of such an event ever been demonstrated?
Plywood sheets can penetrate tree trunks unscathed


The plywood isn't unscathed by the process




  Why wouldn't an airplane [be unscathed by] impacting ocean water at a steep angle at 300-500 mph?


The aircraft will not travel at that same speed underwater. That means that a considerable force is applied to the aircraft to decelerate it in a short distance. That force greatly exceeds what the structure was designed for.