No, that would be a lack of unions that does it.
The single benefit won by unions that managements around the US have been very leery of touching has been the vacation benefit. In companies - they're rare these days, but there's still a few - where unions have won at least two weeks of paid vacation even for starting workers, that automatically means management gets more.
You will notice that if there's a dispute between the union and management, vacation time is not usually the bone of contention. Pay, pension, health care, "flexible" work rules, sick days and whether they accumulate from year to year, overtime pay, holiday pay, you name it, but not usually vacation time. 'Cause, you know, management likes that extra vacation time.