A template is built for nobody in particular. The same design has to suit a florist, a solicitor and a garage, which means it suits none of them properly. You end up rearranging your business to fit the website instead of the other way round.
Building from scratch takes longer. It also means the site says what your business actually does, in the order your customers need it, and it is not carrying code for features you will never switch on. That is most of the reason a custom site loads faster than a template one.
It also means we know exactly how it is put together. When you ask for a change, we can just make it.