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.
There are numerous companies out there providing professional designed web templates. There are also site that offer free web templates. Then off course, one cannot expect all the free web templates available on the web site are professional, there are however, some free web templates are designed professionally and optimized correctly. If one is seeking for more professional service and support, one can opt in for premium web templates company. An example will be DeonixDesign where the company offer thousands of professional templates. DeonixDesign work as after user subscribe their service, they can have unlimited access to all the contents available there.