You don't understand the cost of software development. Yes some trivial app to display a battery voltage takes a few days to write and some kid can do it at home after school. But things like say, a video editor can take multiple man years of engineering time. If your engineers are paid enough to live in a house, own cars, send their kids to collage, pay for health care insurance and so on then a "few man years" adds up to some serious money and next to that $5K is trivial.
The trouble with the app store is that toy size apps are some easy to write than even kids and make them after school so we see a flood of this junk. But look at Apple's list of top grossing apps, most of those are the type where $5K is "nothing" compared to other costs.