While people having skills in HTML and other web scripting languages can opt for designing their own site it is not a viable choice for everyone. Some of the web designers use pre defined and readymade web templates for quick and effective site designing.
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.