$13b is a big-sounding number. But it's not that big in comparison to some other numbers. For example, there were 75b downloads from the Apple App Store last month, so even if that $13b were just for the last month, not for the lifetime of the App Store, it would amount to less than 20 for each download. There are 1.2m apps available, so $13b means just over $10K per app. That's quite a lot for a week's work, but it's a pittance compared to the cost of developing a typical program, especially when you consider the earnings per year.
Oh, and for reference, Microsoft's revenue for the last quarter was about $20b. Which makes $13b spread between 1.2m apps seem very, very small. (I'm assuming that your $13b number is just for developers selling through the Apple App Store. If it also includes Android then it's an even more laughable number).