There is a wide range of CMS‘s available for use. Some of them are free like Joomla, WebGUI, Bricolage, Mambo and some of them are commercial ones like Bitrix, Drupal, etc. Joomla notably stands out from the other CMS‘s thanks to its quality and usability.
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.