Wouldn’t cylindrical be a lot better?
They show cars squeezed down so tight it’s hard to tell what’s what. Better to have distortion in one direction only, to add side lanes to your view, but not a lot of extra sky.
“Distortion in one direction” is a bit misleading. If you shrink an image in both vertical and horizontal directions, it doesn’t distort - it just shrinks. A square becomes a smaller square, not a rectangle. If you shrink something in only one direction, then it distorts (changes shape, square becomes rectangle). A spherical mirror has some distortion, but much less than a cylindrical mirror.
Also, I think you do want a wide field of view in the vertical direction. You’d want to look down at nearby cars, as well as look up at distant cars.
The distortion is not really the issue. The curved mirrors are there to show what is in the truck’s blind spot. You don’t necessarily need to know the size of the car, merely the fact that the car is there.
Those blindspot mirrors (I call them) are essential to us truck drivers. Our blind spots are huge, without those mirrors, we basically can’t drive. We can’t se what’s right beside us, and we can’t see the end of the trailer when we’re turning corners. Also, when I’m backing up around a corner, the little mirror gives me more visibilty than a flat one would. Hope this satisfies your curiousity.