Except when there aren't the trained/qualified/experienced people to do the work. You assume an infinite supply of workers.
That's not the case. Look at the well known shortage of dentists and doctors leading to long waiting lists and delayed treatments.
Likewise shortage of other professionals such as qualified/experienced engineers, actuaries, etc.
It costs money to educate/train people for the senior professional roles and can take years, if not a decade or two, for the likes of medical consultants. We need people like that to work as much as possible and not "encourage" them to reduce their hours/days by a dysfunctional tax/benefits system.
I've been talking recently to three accountancy practices as I'm retiring and wanting to sell my practice in the next 2/3 years. All three are really struggling with getting qualified/experience accountants recruited, and are advertising widely/using recruitment agencies, and offering a decent remuneration package, but the only applicants aren't the level they need. They all say the same - the 40/50 year old staff are going down to part time by knocking 1 or 2 days off, the 60+ year olds are retiring or going down to just 1 day, and in the middle of all that, they're having to cover maternity leave as accountancy has become a popular profession for women. All three firms want to buy my business but all three want me to work for them rather than retire - they've the staff to do the menial/lower level work, but don't have the "top end" staff so independently they're all offering me the same "package" of me going in to do high level work and "attracting" me by taking off my "low level" clients. If I'd have realised what was on offer, I'd have moved over and done it years ago!!