Been trying to figure this out for months, and I think I finally got it.

PEGI, the rating system, has no problem with the nature of gambling, which is the use of an addictive game of chance to extract a ton of money from players who are vulnerable. That's totally fine and in fact gets a three and up rating from them.

What they take issue with is people learning about the rules of pre-existing gambling games. The money loss, addictiveness, and so on would have been fine, but deigning to make a game that references the rules of another game (so people who already know the rules don't need to re-learn those rules and in the process making the barrier to playing this new game lower) is in their opinion despicable if children somehow learned those rules. Not children learning to gamble, as stated before they have no problem with them using parents credit cards on games of chance, PEGI doesn't want children learning the rules specifically. They may claim it's to stop them gambling or something, but as we already know they give real gambling games the a-ok for three year olds, right? So stopping them from gambling is, functionally, not the problem here, the problem is learning rules from games that were used in gambling for a long time.

The part that kept confusing me was I thought, surely if gambling was the issue they'd do something about other modern games with gambling mechanics, but not only did that never happen, but they changed their rules later on in a way that led to Balatro being a PEGI 18 game, so changing the rules has always been an option, they just refuse to.

In short, PEGI loves gambling and thinks people of all ages should be able to throw money away on games of chance, but they hate the rules from old games of chance and so they restrict anything using those rules to adults. Any reasons for why don't apply universally to the gambling games they do allow, and so no reason truly exists, because they changed their own rules and have proven changing the rules isn't the issue here, the issue is that gambling is a big money maker, and they must love it or they would have age restricted it already.