Like all technology, concrete sidewalks didn't always exist. Flip back to colonial days, and you see dirt streets, or maybe cobblestone, gravel, or seashell. What went on the streets was all a function of what materials were available. Somewhere between there and here, someone got the bright idea of a sidewalk, especially once there were cars running people down, and cities thought them a good idea so put them in. They proved rather successful as the boot scraper, which scraped the mud off your boots, became a thing of yesteryear.
Today, concrete sidewalks perform as excellently as ever. Many have their top layer of cement worn off, leaving a slab of bumpy stones to give your feet traction. Literally billions of trips per day walk across this technology. So the next time you want to see a modern marvle, ignore you iPhone. Look down below your feet. It's the sidewalk that takes you everywhere.