Why do most sidewalks have cracks in them every few feet?

It's because of the landscape, it changes over time.

This is actually a really good question! The lines are actually called "contraction joints." When the sidewalk is being made, the concrete used shrinks as it dries. This shrinking can cause cracks to form in the sidewalk, so to prevent the cracks from forming randomly, the builders make thin lines. Any cracks that form should follow the path of the line, since this is where the concrete is thinnest.

Hope this helps! :)