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.
So, more and more sites are operating on Joomla nowadays. Joomla Templates are easy to use and they give you a chance to manage the content of your web site without going into details of the source code. A user friendly admin panel enables anyone to edit texts, pictures, product images, etc. If you wish to have Joomla like a splendid foundation for your online project, you can choose any example of Joomla Templates offered by Template Monster.