Are you talking theme parks, estate parks, state parks, local parks, or federal parks? Of those 5, 3 of them have the ability to do so. Also "capacity managing" a park actually takes more staff than closing it does. If the roadways into a park have gates, and you can manage to 'block' the use of any equipment (locking bathrooms, shutting off water lines, fencing off playgrounds) all you need is an occasional cop to swing by the park to look for trespassers (likely no more than they do on current patrols). Managing capacity of a park means having someone at entrances, counting entrances and exits, having staff within the park to manage flow. On top of that if you hit 'capacity' and people can't pull into parking they are backing up traffic on the road. That costs the entity managing the park much more than normal operating costs, and most boards are trying to cut back on spending.