I experienced this in the extreme recently. I'm a very experienced dev in a very in-demand field. Money is not a huge priority for me in my career. Flexibility is. I have a proven track record of success in part-time roles after converting to part time at my previous company (after 2 years of full-time, of course). I spent 3+ months trying to get part-time engagements, and was willing to consider both consulting and…
Another blocker is the inane requirement in employment agreements by many large corps in the US that they own your IP while you work for them, regardless how it was developed or what the topic is (one place I worked at owned children's books if you published them, despite being a software developer). Until that is unacceptable, either legally or culturally, working part-time presents a conflict of interest. Rather than valuing the breadth a part-timer might bring, it's viewed as a liability.