The biggest reason why they have not done this so far is SEO. If you introduce the 'click wrap' - other crawlers like Google won't be able to crawl it, so their traffic will decrease overnight.
Showing different results to google than you do to users is called cloaking and it's not allowed
https://support.google.com/news/publisher/answer/40543?hl=en
Apparently, we had something similar at Demand Media before the panda update.