Why do improvements and redoes, of something already done, cost extra?

Building a website is a creative process. Frequently placing an order the client misses some details on their vision and the sites functionality when. Also frequently during the work process the client adds to and builds upon their idea with lots of details, such as layout, number of menus, menu functions, additional features and much more. This way something that is done right and setup to work has to be deleted and in its place something new has to be build or something already built has to be edited or improved in some way (for example an online store with Easy Pay application has to be changed with a PayPal application because the client decided that they want to sale online goods not only in Bulgaria but also abroad. Another relevant example is already formatted, mounted and ready pictures where a happy team is smiling to the client have to be changed to neutral pictures for example from the product line of the client. This means the new pictures have to be formatted again, to be cleared, their contrast ratios to be set and to be put in the place of the old pictures.