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.
With website templates you can get the best of both worlds. You can get something that is low-priced, was made by a professional developer, and is going to have at least a semi-professional look that should suit you just fine. These designs have been developed exclusively for professionals and individuals who want a simple solution. You will be able to add your own images, content, etc. to the site. As far as price goes though, it can definitely fluctuate. You can find low-priced templates or higher priced website templates.