I tried to negotiate more holiday time (25 days rather than 10 days) when I was planning to move to the US. They wouldn't budge; but they were willing to employ me as a remote worker living in London, UK, rather than locally in CA. (I didn't move, in part because of this, even though I had a H-1B visa at a time they were handed out by lottery.) So now, I get the extra holidays (but less pay), as well as having frequent transatlantic flights to get up to date with the US folks.

Why was that? Could it possibly be because the willing partners in that private agreement aren't as free to choose the terms of their agreement as you seem to think they are? The extra holidays aren't just a line item in the contract, with appropriately reduced pay etc. They are perceived to cause jealousy problems, for one thing. They're also structured as a status perk for long-term employees ("earn a whole extra day for every year of service!" they said).

The issue is even bigger for someone contemplating emigrating to the US, because longer holidays increase the cost effectiveness of transatlantic trip home. Only having 10 days holiday effectively means saying (relatively) permanent goodbye to all your friends and relatives if you also want to have e.g. a Christmas holiday and a summer break.