Here's why: you've been at company for a while. You know the code, you know the dependencies, you know the business logic, you know who to talk to, you know the mindset, you know the priorities. Replacing you is expensive. Even if they can hire someone with same skills on paper, there's a long and costly organization-specific ramp-up for most companies.
Thus, a good rational manager (not all of them are, obviously) will prefer to have you part-time than to have you leave. Which gives you negotiating leverage.
The longer you've been at a job, the more valuable you are, the better this goes. Even better if it's a tight hiring market, like now.
(I write about this at length in my book, Programmer's Guide to a Sane Workweek: https://codewithoutrules.com/saneworkweek/.)