> Why isn't this the norm on desktop - ad supported but free software? Why aren't there ad networks for desktop apps like there are for mobile apps and web content?

My guess is because the desktop model doesn't assume live internet access. My understanding of ad networks is that they involve a live bidding process against interested parties at the moment you load the page. This allows integration of live geo data, what you just searched, what you just looked at previously, etc. And you don't need to reconcile what was served if an app goes offline then rejoins for charging the advertising account.