The problem is that the operational overhead for two part time employees is much higher than for 1 full time employee
Just one developer means that there are no merge issues due to ignorance of what the other is doing, there is less need for spending time on documentation (foolish over the long term, but most don't see it that way), and requirements don't get mangled further in the game of telephone. Most of the informal processes used to replace a formal project management strategy really begin to break down as the team expands. Many of those processes also assume things like "oh, you can just ask X", which doesn't work if X is out of the office half the time (or if they quit, but this aspect is rarely considered ahead of time) and that leads to blocked work.
Programming isn't like McDonalds where the system is designed to make people interchangeable cogs who can be swapped in and out. Some tools like Scrum try to do that, but even then, a developer has to work on a system to develop a base level of knowledge before they are productive.
A highly organized company could pull it off with just part timers, but companies don't tend to be that way when small and if a company is large, they need a whole developer (or several) anyway.
Another problem is that many companies have the expectation of completely owning you career wise for the time you work for them. Instead of buying 40 hours of your time, they see it as more purchasing unlimited capacity from you with an informal agreement that they won't use more than 50 hours regularly. Part time work is clearly pay by the plate while companies are used to effectively having an unlimited buffet.
The shortage of developers also means that many companies would pressure you to go full time as an easy way out of having to put up another job posting.
Some options for you:
Find a company with a compressed work week. My company allows you to compress two weeks into 9 days or 5 days a week into 4. It isn't technically full time but it may suit your needs.
Work full time and then see if their is a parent track. Certain companies also have this arrangement for parents to try and retain workers who have had the circumstances of their life change. It usually is only available to people who would be hard to replace.
Found your own startup. Plenty of business guys out there with ideas so as long as you don't need cash soon and can wait a bit on a speculative return, this could easily work. A lot of startups don't actually require full time tech work.
If none of those work, you could found a job board for part time technical work only.