The main point is that even the third party plugins haven't gotten this working perfectly yet and they have dedicated teams working on them. If the folks at scirra were to work on this, they would have to basically redo all the work done so far in the third party plugins and then improve upon them to make them even better and more integrated than the companies who wrote the third party plugins. With only a 1 man team working on the C2 tool itself (Ashley does all the work on C2, Tom handles the web site), this would be a pretty huge task. They would also have to keep up with all the changes that each of the platforms makes in order to make sure their functionality always works. They would also have to figure out how to make it work in spite of the third party wrappers that are used to package the games for the various mobile platforms. Basically figure out hot to get it to work with cocoonjs, Ejecta, Crosswalk, XDK, phonegap, etc... if it is truly going to support everyone using C2. It just seems very unrealistic for one person to do. Personally, I think a team of people funded by a kickstarter, or some other method would have a better chance of creating a universal Advertising object. And then maintain it as well. Regardless of who anyone feels "should" do the work, reality doesn't always follow that logic.
Don't get me wrong, I would love to see it, I was amazed they got even a basic IAP object implemented. Though I still have yet to hear if anyone has had success with it across all the marketplaces and/or with all the various wrappers.