It's said pretty plainly that, for whatever reason, they aren't able to eat meat very often. They don't even have the resources to build personal ovens; presumably they roast whatever they forage as quickly and efficiently as possible to feed as many people as possible.
I think the logic is that, combined with unstable living conditions and short life expectancy, knowledge that isn't used daily (like "how to cook meat") gets forgotten easily over a generation. Without trade, the knowledge doesn't get imported either, unless someone deliberately bring it in like the MC.