Because “the markets open at 9” is an international standard that everyone can count on. You could stagger it so that one country’s market opens at 10, then another at 12, and so on, but then what if one country chooses a different standard? What if a restaurant picks a different convention than businesses in one area? Time zones are great because once you understand them, you’ll always know how time works locally, anywhere in the world with a single piece of information, it’s a truly successful standard.