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?
Well, OneBox isn’t really monetized yet. There are sponsored listings around it, but the OneBox area itself isn’t monetized. I think the fact that the pages themselves (regular SERP pages) are monetized makes it ok. So it is sort of monetized already.
Now, what about Base and Froogle? Same thing. They’ve already got AdWords running down the side.
So, for those who were wondering when they’re going to monetize this, the answer is that they already have, although not directly. This is the same way they monetized organic search, and it seems to be working quite well there. I don’t foresee any changes in the immediate future.