Web Development versus Software Development
Some web sites are more like software, rather then a website in a traditional sense. They store data / interact with a database on the back end, they execute some business logic, and process information in a more intricate way. Sure they have an web interface, but although it is the part of the web software application we can see, it is not the most significant part (in terms of programming effort at least). Web development is a job of creating a web-based software rather just designing a pretty interface / website. And it should be done by a professional with background in programming and software development, rather then design.
With modern web development tools and frameworks, one can go a very long way in creation of functionality, without even releasing what goes on behind the scene. Without thorough understanding of software design principles it is impossible to achieve solid software architecture that leads to reliability, scalability and application security.