It's not surprising when they make less than $30 a month.
That's the thing, though. Devs who haven't been developing for long enough to gain a decent following, and are doing it alone, and doing it for money, tend to fail. Devs who are alone, and are just doing it cause they enjoy it won't be hurt by lack of funds, but they'll be slower in the process due to working a real life job. And provided they don't quit due to everyone whining about how slow they are, they actually can finish a project.
$30 a month is actually more than the average no name musician makes from Spotify and other streams. Yet, musicians keep spending time, money and effort on their passion, while working regular jobs. lol