The design was meant to be functional. Tesla wanted to use stainless steel that doesn't need paint, which is a significant part of the car cost.

But stainless steel is much milder than normal steel, so you need to use thicker panels. But thicker panels are more difficult to stamp. So why not then double down on thickness, and make the panels structural? It makes them impossible to stamp, so double down on the "flat" design.

Well, it didn't work out as intended. The whole flat panel look ended up being a total gimmick.