Comment by Centigonal

Great point, and also: the walled garden operators don't make any money off of free apps, so they don't prioritize supporting creators of free apps. The result is that it becomes unsustainable for most developers to keep a free app on the store.

The incentive structure of walled garden app stores encourages the proliferation of paid apps for everything, including things like an instrument tuner or a notepad app that'd be free in a non-walled garden environment.