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.
When it comes to the high-priced website developers, they are going to have a reputation to back them. They are going to have a portfolio of professional websites they have created that are going to speak for their work. High-priced website designers are going to have people who could vouch for them as well as have a very strong web presence. This is the reason why they are going to be priced so expensively, because what you are really paying for is a degree of certainty, not necessarily superior quality.