@givemersspls The deal with plateau is rather simple. They are pretty much all fighters without spiritual energy. You don't have to cultivate to grow physically stronger, but having spiritual energy would increase their battle prowess considerably.
Besides the plateau looks like refuge their ancestors fled to so we can assume they had to leave cultivation manuals behind in a hurry.
@givemersspls The highland people do have cultivation techniques, but they are incomplete. I guess those rare demon spiritualists are simply lucky enough, as you said, to have a compatible (and complete enough) cultivation route and compatible demon beasts that they can catch.
I can't remember the details in the novel for this part, but it's all back to limited resources.
Looks like some kinda Sherlock Holmes chapter. Yeah we get it Author, we knew what happened since it was only one to two chapters ago. More filler padding, oh well.
@Merilirem Everything is filler for them since there is no fighting going on. you can really see this when you have read some of the comments on vinland Sage.(Tough maybe not on this site)