Why change class or interface is a problem?

I read about Builder Design Pattern on this wikipedia link and it tells that

Creating and assembling the parts of a complex object directly within a class is inflexible. It commits the class to creating a particular representation of the complex object and makes it impossible to change the representation later independently from (without having to change) the class.

I don't undestand why create and assembs directly in class is inflexible? With different representation I can add different method for class, why it's not good?

Thanks for all help,