Amazon is already there.
https://finance.yahoo.com/news/amazon-is-quietly-building-th...
Interestingly, Amazon's move to ad placement seems to coincide with how terrible Amazon's search is. It's a pay-to-play free-for-all wasteland. Not too dissimilar to the SEO wasteland of Google search.
It's not just that Google can take risks because they have margins. It's more that they need to take risks to diversify their source of margins before they disappear to someone like Bezos.