I can think of one reason why you wouldn't just want everything public.
For instance, variable you never intended to use outside of the class could be accessed, even irdirectly via chain variable access (i.e. object.item.origin.x ).
By having mostly everything private, and only the stuff you want to extend and possibly refer to in subclasses as protected, and generally only having static final objects as public, then you can control what other programmers and programs can use in the API and what it can access and what it can't by using setters and getters to access the stuff you want the program, or indeed possibly other programmers who just happen to use your code, can modify in your program.
