In my European mind (I have 25 mandated days off per year), if there was not a mandatory paid vacation limit two things would happen:

1. Further exploit desperate people since those that don't need to work at any cost would steer clear of jobs that have 0 holidays. 2. You would further penalize people with families where both parents work. It is well understood that if your kid is sick you can't really use your sick days and so must use your PTO days. Having 0 available days doesn't play well with having kids (personal experience).

And finally, having mandated PTO allow you to actually take holidays. I heard too many times of companies that offer unlimited PTO and when the employer tries to take some they sabotage him/her or plainly threaten his/her job security.