Filtering is the main reason. I've experienced this twice especially. The first is that I'm an embedded software developer, and quite often devs don't know what that is so they'll just send blindly off an application without reading the description.

The second was when we were looking for an EE graduate and specified that applicants must have an engineering degree. We got hundreds of graduates with every kind of degree under the sun who were just blanket sending their CV to every listing with the word "graduate" in the title. With that many CVs we had no choice but to use a recruiter to sort through everything.