Truely excelent question, and I totally agree. But, I'd have to spend huge amount of time figuring out how to model one that looks OK, simulate the physics for it, and then render it, which is a significant investment of time, and I decided against it.
You're actually the second one to ask about it, so I should probably add a disclaimer along the lines of "The makers of this game fully endorse operating moving vehicles in a safe and responsible manner, including wearing of seat-belts by all occupants at all times, it's just they're a beyatch to render so we didn't. Drive safe!"