The real problem with app stores is that selling your software direct to the user is a solved problem. In many casees the e-commerce platform you use to sell your own software is better than the app stores for the seller (and in many cases the buyer). There really isn't much value until you start bundling the developer tools and libraries. Most of those libraries are derivative or are open source. So developers are trading no real (sorry but Apple's $100 is de minimus) up front costs for a percentage fee paid by the customer at each sale. This is probably optimal for small teams and solo devs...