Lots of good points have been made, but here is another one, the cost of making a reservation is often only a tiny fraction of what the RV trip costs, so when one is forced to reserve a site months in advance, being out the cost of the rv site is trivial in the greater scheme of things.
Lets take Yellowstone as an example, if I were to drive there this summer in my coach from Louisiana, my cost to get to the entry gate and back would included spending something over $1,200 of gasoline, take 8+ days of my time even pushing it with long driving hours, realistically it is 5 days there, and 5 days back at around 325 miles per day, so probably another $300+ rv park fees along the way, not to mention food, etc. Making the $35 or so per night at the destination paid out months in advance trivial when considering the overall cost of the trip.
If I were still working full time, I can see how it could make sense to book sites at 2 or 3 different destinations months out and just loose out on the ones you choose not to go to, due to whatever reason, wild fires in the area that year, not being able to take off the extra vacation time, so falling back on the closer to home backup trip instead, etc..