For the same reason we don't have a single 'car standard' where every car sold has the exact same feature set. Same size engine, same tire size, same number of passengers, same radio, same transmission, etc... People actually want different cars. Some want a pickup to haul or tow things. Some want a little sports car that one carries two people. Others want a mini-van to haul their 5 kids or the soccer team around. Yes, some things would be easier if every car had the same parts and features, but most people don't want that situation. Same with your computer hardware and operating system. People want different things.