Destruction is simpler and more operational than creation, and loss is more likely to produce emotional fluctuations than gain. Even now, billions of people on the earth have not yet reached the dilemma of lack of resources. The key problem lies in the uneven distribution of resources. Is the war because there are no resources? Because of the desire for more. Is there a shortage of resources in America? No. But almost all local wars have its participation, so even if Thanos can create resources, it can't solve the problem of greed at all, and destruction can at least reduce a few bad guys at random.