Very few companies are adept enough to do so. Sure, implementing a similarity metric is easy. Scaling is a bit harder. How do you optimize? Are people clicking

Very few companies are adept enough to do so. Sure, implementing a similarity metric is easy. Scaling is a bit harder. How do you optimize? Are people clicking on the links? Are they buying? How many even know they should be optimizing for those metrics, let alone consciously addressing more advanced trade-offs of LTV vs short-term sales? Even worse, with less data there are more problems with every class of algorithm.