I don't know for sure but I think it's the current, higher voltage will bridge a bigger gap/higher resistance but a human body doesn't have that high of a resistance and car batteries are capable of providing plenty of current (I think?)

That’s why it’s more likely used for fetish purposes irl, and even the maybe two in series? You’d have to ask the experts, which I’m totally not one of. Not involved in that world at all. Definitely don’t have a monthly budget for that…