There’s nothing wrong with buying from a stock photographer directly. But you can expect one photog to have less amount of images available, less variety of subjects and topics, and for the operation to be a bit clumsier. Not to mention there aren’t a lot of artists selling their stock independently (they also think it’s best to sell through a stock agency!).  In a stock image website, you find thousands to millions of photos from various artists, all in one place. The company takes care of reviewing the images, managing the licenses, and ensuring they are cleared for commercial use.  As a buyer, it makes a lot more sense to use a stock site.