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.
While making a site using web templates a person can make a base template file where it is possible to specify the non editable and editable regions separately. People also prefer the template based sites as they tend to be search engine friendly if the template is optimized properly. One can take the help of certain web site design software like Dream weaver for creating website templates easily. However, it also helps if one knows HTML to an extent.