Stock photos are photos shot by photographers for the purpose of editorial, commercial or other use and usually marketed through a stock agency. The photographer usually spends a lot of time thinking about metaphors and building a photo around the theme.
Back a few years when stock produced better $sales I spent much of my time creating photos that had potential for sales. Some had good success. Some never made it.
The “unusable stock photos” have problems! Technically most are ok. They are exposed ok and are in focus but they lack in other ways – what do YOU think?!