Because it encourages people to show up to work sick if they have a vacation coming up, or at the very least do a cost-benefit analysis with regards to taking a

Because it encourages people to show up to work sick if they have a vacation coming up, or at the very least do a cost-benefit analysis with regards to taking a sick day versus showing up sick. If you have a sick pool, which most people will never come close to using up, then beings sick just becomes "oh, I'm sick, better take a sick day".