But a lot of these places are already crowded/busy so don't need "more" people, otherwise they may have to start refusing entry, and they'd lose repeat customers if the queues were too long for rides/loos/meals etc. Even with capacity, more people usually means higher costs, such as more staff needed etc.
The bigger places are run by professional/experienced owners and managers and will have lots of data as to their price versus number of customers, so can make informed decisions.
I've worked with lots of businesses, granted much smaller businesses than the likes of Alton Towers, and we've regularly done "elasticity" to find the "sweet spots" to maximise revenue, and in most cases, the loss of custom from prices increases costs less than the extra revenue raised from the slightly lower number of customers, so controlled price increases usually increases revenue despite losing a few customers, and of course a few less customers often means lower costs too, thus increasing profit further.
After all, any fool can make a multi million pound business by selling £5 notes for £4.99 - often fewer customers paying more, sometimes for a better experience, results in higher profits than the Aldi mentality of pile high and sell cheap.
The real skill is getting punters through the doors on the quieter and off-season days, which is where the "free" return entry comes in good, as they don't get entry fees again, but the customers will fill the place, buy souvenirs, meals, etc., and most places also have "chargeable" extras like slot machine arcades, special rides, etc. They don't want your second/third visit to be at peak time, they want it at a quieter time. A free yearly return pass also helps justify a high price for the initial entry as they can sell it as a yearly pass instead of day pass, and of course lots of people won't return, especially if they're not local.