That and because they're more dangerous per kilometer. I can go 240km in two hours in a car, but only 16 on foot, and I'm going to be crossing many, many streets in that distance.
Here lays the body of William Jay,
Who died maintaining his right of way,
He was right, so right, as he sped along,
But he's just as dead as if he were wrong.
Moral of the limerick? Optimize for for your own safety, not mere adherence to the law, because lawsuits cannot bring you back from the dead. The other person being wrong for killing you won't make you any less dead. This is something I try to keep in mind whenever I'm driving, crossing a street, etc. The crosswalk may say I can walk, but for my own sake I better look both ways anyway.