Justin, that's a good question. I am sure there is a valid reason that probably has roots in commercial use-and that's why you only see them offered on 3/4 ton+ trucks.
But you mentioned other than on a semi, what is their use. And then mentioned that the headlights on a pickup are wider apart than the headlights, negating their use for indicating width. That is also true for conventional-cab semi trucks, because their fenders are wider than the cab. The only ones that would be applicable to are cabovers. And nobody really sells a cabover in the USA anymore. Only Freightliner offers one, and it's only a glider-they don't sell a complete truck. I'm not sure why even a conventional-cab semi needs them since most seem to have stacks well above the cab.