Because Google makes money from ads, they're not actually optimized to build the best general search engine in the world, they're optimized to build the search engine that makes the most from ads, which is correlated with being a good search engine but not perfectly aligned. Our business model (paying directly for the search) incentivizes us to try to return the highest quality results, without any bias toward making money from ads. It also enables us to do things like pour a ton of compute/resources into a query to get the best possible results we can find, because someone would pay us a lot for that, and that's hard to do under an ads-based model.