Nothing is preventing that.

That's what a lot of GPL advocates are missing here. The GPL doesn't specify how things are supposed to get done, only that they do get done.

The GPL boils down to a couple of things: Make the source available and allow others to modify and redistribute it (as long as they follow the GPL).

Nothing about bundling the source with the app or having access to the same avenues of distribution as the original author or even the same tools.