They're not car roads. They're roads.

In a lot of places it's illegal to ride a bike on a sidewalk. It's dangerous and rude, pedestrians shouldn't have to dodge bikes riding around on the sidewalk. That's a place for walking, or at least for moving very slowly around people who are going to walk slowly, stop without warning, turn, behave erratically, and aren't expected or obligated to hold their line or move predictably. The road is for wheeled vehicles and any kind of speed, it's also a place where everyone is supposed to move in a predictable way, go and stop when they're supposed to, signal turns, and other things like that that make it possible to move at speed. These are some of the reasons it's usually safer to be on the road than the sidewalk.