Why does the C++ linker require the library files during a build, even though I am dynamically linking?

The compiler isn't aware of dynamic linking, it just knows that a function exists via its prototype. The linker needs the lib files to resolve the symbol. The lib for a DLL contains additional information like what DLL the functions live in and how they are exported (by name, by ordinal, etc.) The lib files for DLL's contain much less information than lib files that contain the full object code - libcmmt.lib on my system is 19.2 MB, but msvcrt.lib is "only" 2.6 MB.

Note that this compile/link model is nearly 40 years old at this point, and predates dynamic linking on most platforms. If it were designed today, dynamic linking would be a first class citizen (for instance, in .NET, each assembly has rich metadata describing exactly what it exports, so you don't need separate headers and libs.)

Raymond Chen wrote a couple blog entries about this specific to Windows. Start with The classical model for linking and then follow-up with Why do we have import libraries anyway?.

To summarize, history has defined the compiler as the component that knows about detailed type information, whereas the linker only knows about symbol names. So the linker ends up creating the .DLL without type information, and therefore programs that want to link with it need some sort of metadata to tell it about how the functions are exported and what parameter types they take and return.

The reason .DLLs don't have all the information you need to link with them directly is is historic, and not a technical limitation.