Nothing is keeping them from doing it. They (and others, e.g. the Brave browser and Optimal) have been doing that for a while. Here are a few related links:

https://www.theguardian.com/technology/2014/nov/21/google-co... http://tech.firstpost.com/news-analysis/google-relaunches-co...

https://blog.brave.com/introducing-brave-payments/

https://medium.com/@robleathern/why-we-sold-optimals-technol...