I was actually making a more generic comment: Addams' stuff was preserved, unlike, for example Golden Age OA.
But regarding your examples, even there, scarcity plays a role. An early Picasso from his Cubist period will be worth a lot more than one from his last years. And while there is a lot of Kirby art, how many Golden Age Kirby Captain America pages are there as compared to his 1970's work? In both cases, the price point is higher, but it is still generally relative (once you account for things like special issues, first time appearances, etc.).
By the way, I think an investment in a Schulz page is more likely to have long term growth than most comic art because it has been widely accessible for years and is still being run (or rather, Rerun, pun intended). But that's for another day.