Maybe companies are afraid of handing too much bargaining power to the employee: 20hr weeks basically mean you can get a whole second 20hr job. After a while of truly getting to know both jobs, you will likely leave the worse one. You could do this iteratively. (Use the 20hrs that you just freed up to find yet another job, then quit the worse one again; rinse and repeat until you have a truly great job).

This would be a situation akin to what an economist talks about, when they talk about perfectly competitive markets (driving profits down to zero/"normal" levels).

What we have now, with the market offering almost exclusively fulltime jobs, is actually markedly different: Whenever you switch jobs, you're, to some extent, buying a cat in a sack, taking a risk that the new job will end up being worse than the previous one. The risk is a barrier to entry, akin to what an economist talks about in connection with limit pricing. Here a market incumbent can charge a premium over a perfectly competitive price, because a would-be market entrant cannot sustain enough profit on the perfectly competitive price to also pay back his initial investment of market entry.

Or another point, based on a thought experiment: Say you are staffing a company purely with 20hr employees who are using the other 20hrs to have a go at starting their own businesses. Assume, further, that the personal fulfillment of having a successful business of your own is something you can never hope to match for your employees, so that everyone who is successful will leave. Your company is now staffed exclusively with employees who are bad at entrepreneurship. -- This is vastly oversimplifying of course, as you can just fail at building a company by being unlucky rather than intrinsically "bad", but you can see how it acts as some kind of a negative selection effect that works against the company offering the 20hr deal.