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.
These days internet has evolved becoming part of our life. Every successful business and company needs a website. Even individuals need their own sites for interacting with people. However, for attracting customers to site and making the viewers interested one needs to pay importance to web site design. The aesthetics of a site play a great role in ensuring its popularity.