Why a Crate Does Not Move
A ship rolling in heavy weather tips further than most people imagine, and everything on board tips with it. A crate that slides across a hold is not merely damaged goods. A few tons moving to one side changes how the ship itself sits in the water, and a ship that leans a little leans further the next time.
So cargo is not simply stacked. Every layer is tied down to rings set into the deck, and the ropes are tightened until the stack and the ship are one object. Nothing is left with room to gather speed, because it is the speed and not the weight that breaks things.
The old way of showing this to a new hand was a bucket of water. Carried carefully, it rides quietly. Given one small push, it sloshes, and the sloshing grows with every step until the bucket is hard to hold. A hold full of loose crates behaves the same way, at a scale nobody can hold.
Modern containers lock into the deck and into each other, so a stack of them is bolted rather than tied. The reasoning has not changed at all.