The problem is that at such high velocities, during the impact everything acts like a fluid. Tensile strength and shape is almost irrelevant. You can think of the atoms in the armour not as a solid but as individual particles just hanging in space. Their mass — and hence inertia — is more important than anything else!

Modern armour design is about trying to have moving components that present “fresh” volume/mass to increase the effective thickness. Either via rubber layers that can distort, moving steel plates, or explosive reactive armour that do the same thing but faster.