Because most of the different countries are super old, so they already have their own way of life. They aren't just going to sit down and agree to make everything the same when each country has a lifestyle that makes sense for them. And depending on things like the environment or climate, some things make more sense in certain countries than others, so trying to come up with a system that works for everyone in the world just wouldn't work at all!

Even individuals have their own needs, right? So trying to account for the exact same set of rules for everyone in the world is totally impossible.