Because the particular style of OO you're describing has turned out not to work very well. It turns out that often you can't effectively encapsulate state in a

Because the particular style of OO you're describing has turned out not to work very well. It turns out that often you can't effectively encapsulate state in a class such that no one else but the class itself needs to know about that state. As the size of a class grows, mutable member variables approach globals. If you wisely break the class into smaller ones, classes will still contain instances of other classes, so to reason about the behaviour of your class you often have to understand the possible state of its sub-instances. This is especially true if multiple classes reference the same instance.