Went back to chapter 1, and yep, the absolutely very first thing she does when going collecting is crawling below the black wall to the deep layer lol.
Maybe the monsters still try to be efficient with energy since they were originally plants and animals. Flying over the wall is too much energy when eating stuff around them is easier.
To be fair, we haven't seen any flying Strangers yet so I guess we don't know if they exist. Though there was the one buried underground that Kino brought out to be filmed