Who would have thought they would be worth anything? The only reason they are is because certain people talked them up and then certain other people had to have what those first people thought was 'good'.
We might have guessed from them being favoured cards of Vernon, Marlo, et al., but you're absolutely right, you don't know for definite and if you want to take up investment in collectibles as a hobby, the gamble is part of the game of it. Maybe you don't like gambling, but that doesn't mean other people are "ridiculously stupid" for holding a different opinion.
Let me clarify. There are three circumstances under which you'd want to buy rare decks of cards (or any apparently "collectible" object), and all three are equally valid:
1. You want it for it's utility.
2. You're betting on it becoming more valuable in the future.
3. You appreciate it as a beautiful or valuable object in a way other than monetarily.
Some people may consider only one of these to be a sensible reason for buying something expensive, but others may disagree. Personally, I've never spent a lot of money on a painting to hang on my wall, but I know that people do and I don't consider them stupid for doing so, even though there might not be any practical utility to owning the original painting as opposed to a print or reproduction. If you don't understand the mentality of someone who wants to own a rare object for it's own sake or for it's potential future resale value then that's fine, but it doesn't make others wrong.