Yes, it is how it works, but it's not just how it works. There are reasons for the rules, and underlying models for the rules. It's not arbitrary, it has evolved over centuries to have purpose.

It's not "just" how it works. When you have the equals sign between two expressions you are saying that instantiation of the variables must result in quantities that are the same. When you modify one side then you must make the same modification on the other side in order to retain that property. there are reasons for things to be the way they are.

It's not "just" how it works.