These backgrounds are commonly used for portrait photographers. The studio photographer will take the pictures indoors with their lighting and Photoshop their image into these backgrounds. These are good sellers on stock photography sites because not all photographers have access to nature backgrounds. If photographers live in a city it might be hard for them to find these images on their own. It is also easier to find thousands of these images on stock photography sites. With that said, there are now millions of stock photography images now for sale.