Professionals working for bigger companies who build apps for millions of users or on commission for businesses get paid pretty well. But for people working alone on in small groups, developing apps for smaller crowds, the income isn't all that good, because they are competing with hobbyists. Another factor is the size of the market: in principle it is nice for any developer to have a market of 10s of millions of potential customers, but in practice it alters the economics and customer expectations to their disadvantage.
I have an app on the app store, which I sell for $4.99. It sells reasonably well at that price, but if I look at the income it generates versus the hours I put in developing it, I should charge something closer to $39.99 at the same sales volume, in order to arrive at a decent hourly rate. At the same time, customers ask me why I don't shell out for professional artwork, a UX designer, and better support. Other apps offer all that for *free* or for a buck, so why not expect the same from my more expensive app? Simple: the outlay will never cover the little bit of extra revenue it might generate. Those numbers work if you sell a $.99 (or ad supported) game to 50 million people, not if you sell an app to serve a niche-within-a-niche. But both apps are judged the same, and anything over say $1.99 is perceived as "expensive" (which is a joke if you're willing to spend $899 on a phone).