The points about efficiency and overhead (both bureaucratic and in the N^2 interpersonal effects) seem valid, but here's a more jaded take: employers of coders don't want any limitation at all on the hours they can ask of you. They're used to being able to ask developers to work 50, 60, in my experience as much as 80 hours in some weeks for no extra pay (and on hourly contracts to lie about your hours, or it's go look for another job...which eventually I did do, but many can't or in any event won't). Someone who starts by limiting their hours to a nominal (say) 24/wk doesn't sound like someone who will be willing to work 40 for the same wages when asked to so do, for reasons decent (deadlines) and indecent ('We need the shares price to rise a few pennies for a few weeks, so let's fire a few people and let the others, cowed by the firings, take on the work their former colleagues performed, ').

Note: if you've a reputation as a wizard, you can in fact get part-time work, as some will feel blessed that they get any of you at all---this is related to why rock-stars tend to have a lot of casual sex (because they can, because enough potential partners value such so, and because social norms are actually enforced by allowing exceptional people to violate them---'Sure, Steven Tyler [or rms] can act that way, but you're not in his league, don't get uppity,').