Hills & mountains vs valleys & plains. If a resource is found on a plot/tile No matter the terrain, do you Not think the people living near that resource won't migrate to it?? Really? That is Unrealsitic.
Resources have an active countereffect to any resistance to culture after DH modified it to a while back in RCS. Thus, if there truly is something of known value there to be had, then yes, people are going to tend to want to move there to claim and collect it. The jungles deep in the Amazon hardly have anyone living there but there's quite a bit of residency in Gianna where there is open gold mining on the surface regions.
And then tell me why there are so many cultures that live in Jungles even in this current time. And why so many ancient ruins from past Civilizations. The Jungle has much more to offer than a plain/grassland.
Many of those cultures are hiding from the rest of us and are so 'out of the way' so deep in the jungle that they were where all other modern men would not even wish to tread without roads and modernization penetrating their security thanks to the difficulty jungles really represent to live in. They're also way behind us technologically because the act of survival being such a constant effort kept them from great progress.
The ruins we find from past civilizations are from where jungles quickly reclaimed those ruins and until somewhat recent explorations had (and still many do) remained lost to us once people were either run off, killed, or moved out for whatever reason. Jungle regions are usually developed where people do slash and burn to then settle there, but such areas are also quick to recover if left alone and the soil isn't washed off and the place becomes somewhat barren to life which is happening a lot in Brazil.
TBH, Jungles are terrifying places that represent some of the hardest terrains to survive in. Yes there are many possible treasures to be found there in food and so on but the life there is competitive and will often happily kill you. The venemous snakes of south america terrify anyone with any sense. And if you're trying to compare to the forests of North America, I don't think we're seeing Jungles to mean at all eye to eye. Malaria, ants, jaguars, serpents that hide in trees to drop down on you because you're warm walking under them, spiders the size of small dogs, so on and so on... yeah - nope. Not my ideal place to live thank you.
But I suppose that's just my city-dwelling nature even though I LOVE getting lost in deep wood forests that don't cause you to break out in hives and as much as I do HATE the cold, I'd rather live in the wilds of Alaska. I mean, watch Alone or Naked and Afraid some to get a real sense of how challenging jungles really are. It creeps me out just thinking about it.