We like knowing we have a reservation so we don't waste a 500 mile drive
Who does 500 mile drives? We averaged 189 miles between cg this summer and anything over 200 miles was a 'long drive' day for us. And we operate backwards from you, we plan a day's trip and then look for cg at the end of that day. Not the other way around unless we are going to a NP or some other major spot that we wanted to visit.
And we spent almost a month in CO, 2 weeks at South Fork, about a week near Gunnison and a few days at Grand Junction. Not a single full cg in sight. And we camped in south west WY and northwest WY, no full cg there either. Sure if you camp near a major city on a weekend you will find full cg. And maybe closing every other site in a cg might cause it to be 'full'. But that's why we avoid camping close to major cities, unless it's an overnight transit spot. And as noted above we avoid the dark blue states for the obvious reasons.
So I do not know how we avoided all these full cg, we do not camp at pricey resorts for one thing, but I do pick spots that have good ratings on CG reviews.