They participate in the damages
The main reason, which I have read about in some books, is that they do not permit industrial building on those islands, so who would want to live there? It is a very bare bones society.
Answer:
Because it's cheaper to import some foods than to grow them locally (even if they grow well here), and the population is too large to be supported entirely by domestic production.
Barbar or to be specific it is Tuareg.