If you are valuable enough your employer probably doesn't want you to take any vacation. Your only real bargaining chit is being able to get a job elsewhere which means that if your skills are specialised enough and your employer is the only game in town where you would be that valuable then you actually have little to no bargaining power.

Also when you have a large number of staff who all doing essentially the same job they will usually get roughly the same vacation time or that would be regarded as unfair.

Just because somebody works as say a programmer but doesn't happen to be numero uno rockstar-ninja coder doesn't mean they don't deserve some time off every year.