Also, I'm not entirely sure your case is proof that there isn't a big supply/demand difference. I would be very unlikely to hire a developer 10 hours per week because software is collaborative intellectual work, and I'd have a hard time using somebody who wasn't around for much of the thinking and constantly needed to be brought up to date. Their efficiency (in terms of hours of actual coding versus hours of communication and keeping up with codebase changes) would be low, but their cost in terms of time spent by other workers would be at least the same.
Occasionally I'll see small units of work that could safely be done by part-time contractors working on something solo. I definitely looked for this at my last startup, as my cofounder was correctly eager to accelerate things. But it was pretty rare that I found things where the cost/benefit ratio was at all decent.
On occasion I've tried to pick up side jobs for 10-20 hours per week for a second company, and I've always been refused, not because I wasn't technically qualified but because they just weren't interested in considering non-standard work arrangements. If there were TRULY such a huge gulf in supply/demand, I would've been able to get a side job at 60-70% of my normal hourly-equivalent rate, for 10-20 hours per week, is my thinking.
I've also observed a lot of stubbornness in salary bands and so on.
By the way, I speak as a person who has previously made in excess of $200,000 per year in non-stock compensation.