Conventional economics would say that paid vacation will be factored into salary anyway so arguably not having mandatory paid vacation allows an employee flexib

Conventional economics would say that paid vacation will be factored into salary anyway so arguably not having mandatory paid vacation allows an employee flexibility to trade a week off for more money and doesn't provide any negative effect. But as I understand it US companies don't have to offer any vacation time at all to employees, how common is this? I suppose it's also possible that employees underestimate the importance of taking vacation on their sanity so forcing them to make the trade is a good thing.