The different divs are created to allow for the different design options. Since the label, error, fields, and columns can all be positioned and styled individually, they need corresponding html elements.
A form builder form can switch between designs by changing the .css alone, in order to account for a diversity of designs, the html used needs to account for the options available.
What features of the form builder are you looking for? Server Validation and Spry validation can be added to existing forms using the Form Builder Server Validation Server Behavior on an existing form and applying Spry Validation through Dreamweaver is a valid way to accomplish this with existing forms.