It all comes down to how and why stock photography exists.

Stock photography is a wonderful resource, and is a good way to add high quality media to any design project. However, most stock photography is work that a photographer thinks will not work for a given project they are working on. In addition, the main stock photography libraries are used by everyone. This means that whatever images you find in a stock photography library may be high quality, but they probably won’t directly represent your voice, and they will never be unique.