Bingo. The fact is certification still isn't accepted with older collectors and dealers. There may be runs or packs of literally anything you can think of in a holding just waiting for the day the owner croaks and the accumulation heads off to the plastic factory.
In coins later date wheat cents (56-58) bring STUPID money in 67 or above. The population isn't there. However, there are still bags and rolls by the millions that haven't been graded. One sneeze and they will go from $900 to $19.
Grade chasing is a scary game and only for the brave. Nationals aren't much different. The demand rate is lower than most other types--you have to have people hot for a certain area to really make any money. On small size there are more people doing it so more demand so you have somewhat of a cushion. Bragging about a unique National is setting yourself up for heartache. A town in Iowa came out in 2016. Note sold for over $20K! INSANE. Year later Stack's sell ANOTHER from the same town for like $15K. Well the first example went to a longtime collector because that is his hometown--I would've done the same if I thought it was my only shot. The second went to the museum. So now two of the biggest players for that town are out. If another comes out it's maybe a $3K note because it isn't on a radar anymore. I've talked to the guy who bought the first one and we both agree there will probably be 7-12 known in the next few decades based on knowledge that has become available. Hell I have a unique note in my collection that's brother I visit yearly....
That's why I never collect to profit. I collect for fun and that's all.