In summary: It's difficult to monetize an extension that mashes up A and B, unless you own A or B. (And usually just B - RottenTomatoes in this case - because Netflix could have done it themselves.)

The main attempts have been by messing with web pages, e.g. injecting affiliate IDs onto Amazon links or switching Google search results. Most of this violates advertising/affiliate agreements (Amazon's not going to pay for links which users were going to click on anyway) and often diminishes user experience.