I don't think anyone can fault a car manufacturer for this issue. Long extension cord with too small gauge wire will cause voltage drop. It's the same for any electrical load.

Actually now that I think about it, I guess they could include dynamic power factor correction in their charging hardware and software. But it seems like a silly extra expense just to correct for someone buying a cheap extension cord. I think my 100 foot 10awg cord that I used to power the neighbor's stuff after Hurricane Sandy was under $100.