TBF to the people contradicting you: I think some peoples (sic) stuck to slash-and-burn for way too long. Certainly in Finland: lots of forest, few people.

Maybe they even saw that other plants will grow on the abandoned fields but had no use for them. Maybe they even realized the potential for crop rotation but had no alternative crops to rotate with. Extreme poverty and the continuous need to get as much nutrition as possible from the ground might have been a factor.
And remember, the potato is a fairly recent addition.

Also, any form of agriculture is civilisation.