The first reason would be because hiring a web developer or designer is going to consume far more time than getting website templates. Think about it, you have to explain to the web developer exactly what you want. You have to go over it with them again and again and again. You will have to monitor what is being created and make adjustments during the project. Once the project is finished it is probably going to be behind schedule, and adjustments will need to be made that might cost you more money and more time.
If we directly insert the content, we will create a new page template for each page of the site. But this is fine since we are not duplicating the header, footer and sidebar. To reduce the number of page templates, we can store our content in a database and use a page ID to identify which page content to insert. This ID is usually appended to our URLs of the website. If we do this, we are creating what is known as a Content Management System (CMS). Where the theme is likely to be changed often or people are encouraged to design new themes, a separate directory can be created to hold the various themes. A read me file could be included to credit the designer, or this information could be added to the start of the style sheet as a comment section.