That's a lot more work on the developer to do that. They're going to get a million unwanted pull requests, code bike shedding, and issues raised. And they're also going to get dozens of people putting their own copies of the same app on the store to try to undercut them.

So why would any sane developer do that? Getting people to spend a few dollars on an app is already no easy feat. Most users think a five dollar app is outrageously expensive.