>it will explode after it does because of physics

Is this just a fancy way of saying lightweight fragile aerospace tube with hot stuff all over the outside and flammable stuff on the inside will not respond well to falling on it's side or is there some other thing going on here I don't know about.

Sort of? Everything is weight. And since it isn't a design feature to lay down in water there isn't any weight budget for making that modality survivable? So typically when it falls (and this thing is really tall) the interior tanks detach from their mounts and hit the inside walls and rupture. The skin is still hot, oxygen and remaining fuel mix and it goes boom. This time we had fires but no boom. I look forward to hearing what was different.

No, that's exactly what happens. Most of the time.

There was a couple of times when SpaceX accidentally soft landed a Falcon 9 first stage into the ocean, and had it just float around. They're yet to do that trick with Starship though.

EDIT: Of fucking course.