Web agencies often offer a wide range of services, including the creation of both physical and digital marketing materials, as well as various solutions to increase a company's visibility online. We believe that each of these solutions requires genuine expertise.
While a webmaster/web developer could effectively build a website using a CMS or their technical programming knowledge, when it comes to design and natural search engine optimization (SEO), the involvement of a graphic designer and an SEO consultant would be necessary. Similarly, a Community Manager is not a Traffic Acquisitor, and so on...
Certainly, there are often complementary domains, and an individual may still master several subjects. But as they say, to each their own profession.