When I as a child, I was struck by a car while riding my bicycle. I was riding against traffic, and the car that struck me as making a right-hand turn from a side street. They were looking left at oncoming traffic, and not to the right, where I happened to be at that moment.

It's ingrained in drivers to pay attention to the source of traffic in such cases, a sort of optimization tree for their merging decision. If you're traveling against traffic, you're an outlier not considered in others' equations, and you're at risk of injury.