I assume that Tesla set the following requirements that the Tesla Cybertruck design must satisfy:

Tough exterior.

Low aerodynamic resistance to allow long-range and high efficiency.

Large and comfortable interior.

Variable ground clearance, low for ingress and egress and highway driving and quite high for rough terrain.

Good storage, bed and towing capacity.

Battery capacity to provide long-range and high efficiency.

Item 1 above is best achieved by thick hardened (cold-rolled) stainless steel. Such steel is difficult to stamp into curvature form. So, such a tough exterior is usually composed of flat sheets. Since the Cybertruck's exterior is made of hardened stainless steel, it has flat panels joined by angles.