Because there was no reason to divert men and equipment from fighting actual enemies in order to run off on a wild hare chase similar to a bird dog leaving its point to chase a squirrel.
With the Germans track record of occupation it would have shortened the war. Not only would it take a lot of men and equipment hey got less out of France and Russia after they invaded them than they did by trade before.
You have a better chance of getting it if you pay, if you steal the products of a mine, its hard to get anyone to dig more stuff up for you. Same with oil fields and pretty much anything else.