Why hasn’t Google monetized this yet?
The quick answer is their slogan of “Do no evil”. Charging for something ranking by algo would be evil IMO. Some could argue that AdWords is charging by algo, but in the end it’s whoever will spend the most money and has reasonably good content goes to the top. It’s called advertising, so it’s a cash algorithm. 
But what about OneBox, or Google Base, or Froogle?