> There is something that make me think something is fishy in that approach : why are more and more places switch to unlimited PTO if it’s that costly to give folks time off
"Why do people get more time off now than ever before in human history (which they do)?"
There are now more forces than ever, that work against unlimited exploitation of human labor. For example, human beings in most countries have enforceable rights and they want time off, and the market that has to compete for those people. Humans are shitty machines. If you piss them off, they do shitty work. If you treat them poorly they leave. (In reality, "treating them poorly" translates to "treating them slightly less good then someone else") Since having someone quit working for you is pretty fucking expensive (assuming, of course, that they are actually being productive), you better make sure they do want to continue working for you and so you make it worth their while.
Another important example would be worker protection laws, that simply dictate certain amenities.
"Why is everyone doing unlimited PTO, specifically?"
Less clear on that one. Part of it is certainly it being a trend. If other people do it and employees want it, then you might have to go along, again, to not get pushed out of the market.
But since it clearly does not ever actually mean "unlimited PTO" (you'd be out of a job pretty quickly if you tried), an interesting followup question might be what the fuck it actually means and who benefits? Maybe employees are a little confused about how much time off to take and take less than before? Or maybe they feel pressured to underbid each other? Maybe it's also just that the flexibility it adds to the job leads to more loyal employers, which is pretty nifty in a market starved for workers.