Most devs probably also realize they're not going to win the Patreon lottery and become the next Dark Cookie (Summertime Saga). Sure you can make good money if you end up with a really popular game that people support. You could also end up with a game that makes less than $1000 a month or something Patreon's censors decide violates their rules, in which case you get $0. So I don't think we've even scratched the surface of what the market could sustain if there was a more viable place to actually publish your work and an easy way for people to find it without worrying about crawling through malware infested porn sites looking for it.Those pulling in 15k are very rare. No idea where you got most from. 1k is about average.
Earning 2k on Patreon doesn't mean they get 2k. Patreon takes a cut of that. People also have bills, food, assets to buy and other shit to pay out for.
These are also amateur devs that don't work on contracts. These games are something they have a passion for even if it's just a hobby most of the time. Hiring others is a risk.
The chance of you making 15k making a game is very slim but making it in a normal job would take a specialisation and years to get to that stage.
Most of these devs also have other jobs and families. They don't take it too seriously.
I think most of the devs who could make a game that was good enough to justify $10k+ per month on Patreon by themselves probably already have a full time job where they make that much or more. Most of them would take a big pay cut trying to shift to full time adult game development. Being your own boss would be a nice upside but I would bet that it's usually a pay cut. I suspect most people do it as a hobby or a side job, not a full time job.