International house ads publisher. Why not?

This is not really a question but a muse about what I'm thinking of a better way to monetize mobile traffic via house ads. Yes, I seek your opinions.

If I were to have a "worldwide" popular app at hand, why wouldn't I become an ads agency myself?
For example, I can display a banner in my app with contact info so people can contact me directly to publish their ads. I got ads revenue, e.g., on a monthly basis and the channel for paying advertising is by PayPal. So no worry about ads' tapping/clicking, eCPM, etc.

From an advertiser viewpoint, this approach gives a lot more control of how ones would like to place their ads on which apps. Isn't this smarter spending? If more apps adopt this approach, it would certainly impact advertising industry as a whole.

I see a great opportunity for popular apps to monetize in this way, but haven't seen ones. Why? Are there any complications or flaws about this business model?

I know that this is possible if we target house ads locally (e.g. within one's own country), but I would like to expand the possibility worldwide, hence "international" house ads.

I guess you could try it and see how it goes. The downside seems to be that it becomes harder for advertisers to discover the opportunity to advertise on your app. Advertisers in general are looking to advertise their stuff on any app or site that's relevant, so they go to (say) Google AdWords and enter the ad, and then it's displayed in a bunch of places -- your app and others'. If the option of advertising on your app is only visible in the app and other places where you explicitly promote the opportunity then fewer advertisers will know about it. It also becomes slightly harder for them to set up the ads -- they probably already have an account with AdWords or whatever other agency they're already using, so it's just a couple of clicks -- whereas if you're managing it yourself then they need to start a new direct relationship with you.

On the other hand, if many of your target advertisers already know about you, perhaps because they're already using the app, or because it's a small niche that you dominate, then doing stuff directly might work. And it should certainly bring in more revenue for you if it does.