Flat panels cause problems but they're not insurmountable, they just need to be taken into consideration. It's going to be more expensive to make them flat because you'll have to include more material behind the panels. In a sense they cease to be structural and instead are more decorative.

From an engineering perspective it's a horrendous choice, but a perfectly valid one from an aesthetics perspective and it's far from the first time some designer has made a decision that the engineering department has cursed them for.

I imagine the real issue here is that Musk or the upper management at Tesla is trying to penny pinch and is unwilling to make either design concessions or to pay for the engineering time and materials necessary to fix this right.