That's just decision making under uncertainty. I was talking about proper gambling, such as buying lottery tickets.

I still have no idea what your point was. "proper junk food" didn't exist in the ancestral environment; "proper pornography" did not exist in the ancestral environment either. So what?

My point is that you need some high-level ("System 2") processing to associate the action of buying a ticket to the scenario of winning vast riches

Do you need System 2 processing to associate an erotic story with sexual release? To associate the words "Coca Cola" with a nice sweet taste?

I do speculate/daydream about fictional scenarios, and I find it rewarding (I used to that more often as a child, but I still do it). Therefore I suppose it is possible to counterfactually pretend to having won the lottery using suspension of disbelief in the same way as when enjoing or creatiing a work of fiction. But in this case, you don't actually need to buy a ticket, you can just pretend to have bought one!

Well when you were a child, did you play with toys, for example toy trucks ? And was the play more enjoyable if it were a somewhat realistic toy truck as opposed to, say, a block of wood?

Habit created by social conditioning looks like a plausible answer.

It's not very plausible to me. For example, if it were credibly announced that all of the winning tickets for a particular drawing had already been sold, I doubt that occasional lottery players would buy tickets for that drawing.