Having a car written off when you need to essentially take the entire thing apart is fairly reasonable, as is not wanting to repair the actual body part itself.

When the car is made out of multiple different parts, a low-speed bump into a bollard or another car might result in the bumper needing to be replaced. Not the cheapest repair, but definitely quite doable and probably not worthy of a complete writeoff. When the entire car is made out of a single cast that very same bump results in the entire body needing to be replaced - and that's just not going to happen.