Driving is a regulated activity. You require a license, vehicle registration, and (most places) insurance. The police have broad powers that are meant to enable them to make sure drivers are operating their vehicles within the bounds of the many laws and regulations that govern the operation of motor vehicles. Checking up on these things will also reveal any outstanding warrants for the registered owner. If you're on foot, you are not participating in a regulated activity, and most jurisdictions do not require you to identify yourself unless the officer has a reasonable, articulable suspicion that you are, have, or are about to commit a crime. In the US, this is because one is (supposedly) protected by the 4th ammendment to the constitution, which protects against unlawful search and seizure. Other democratic countries mostly have similar protections.