Typically most car body panels are being made via series of giant steel stamping dies that are huge and extremely expensive to develop. A huge hydraulic press applies massive forces to two or more pieces of stamping dies to turn the flat sheet metal into a desired shape. The cost of creating each of these dies is so high that it would be a disaster for the company to use a 3d model whose surface continuity is compromised. Since the car body panels are so big, it’s very difficult and slow to manually fix any imperfection in the continuity of the stamping die.
On the other hand, manufacturers of small electrical devices such like hair-driers make them via steel or aluminum moulds that are initially CNC-milled to a slightly rough state with visible milling markings, and then manually polished by workers with a number of different sandpaper grit. During that process, any imperfection leaved by the CNC-malling that was caused by the 3d model’s lack of perfect continuity could be fixed to a certain degree, because the matrix is small. Lastly, a polishing paste is used to give the matrix a final touch of smoothness.
Also, keep in mind that many plastic devices have a matte finish or a texture (created by using either sand blasting or chemical etching), so their 3d model don’t really need to be 100% perfect.
PS: On a side note, the Sony Dual shock 4 controller is a good example of a badly designed shape. It has plenty of G1 fillets that look especially bad upon looking at them towards a direct light source. There is a visible lack of continuity in several areas. To this day I still can’t believe that “Sony” approved such an imperfect 3d model that was intended to be used for the manufacturing of a plastic with semi-gloss finish (not to mention that later they also released a few modifications with glossy paint). A G2 blend would make it look sooo much better…
