As well as those methods you should give your class a constructor. That will insist anybody using it pass a size to your Box. You should also return the size from the get method rather than displaying it.
In essence the answer to your question is: Because that is the way it is intended to work by definition.
Variables and methods NEED an instance of the object to exist.
The computer won't allocate space to store a "length" until you create a box.
Static methods are class methods. You call them on the class directly, and they don't need you to create an object first.
They are actually the exception to the general rule.
Because you are in a static context (the main method) then you will need to create an object to call instance methods.
A common pattern is to do that as the only thing in a main method because working within that static context can be very limiting.
If you are using a rototiller, you are doing it wrong. Even on this tiny ad: