Our forefathers came to Eretz Yisrael — blessing came in their wake, viz. (Devarim 6:11) "And (you will find) houses full of all good that you did not fill, and hewn cisterns, which you did not hew, etc." R. Shimon b. Yochai says: If it is written "houses full," "hewn cisterns," why need it be written "which you did not fill," "which you did not plant"? (To indicate) that it is not you who filled or hewed or planted, but your merit.
And thus do you find that all the forty years that Israel was in the desert, the men of Eretz Yisrael built houses, and dug holes, pits, and caves, and planted all kinds of fruit trees, so that when our forefathers came to Eretz Yisrael they would find it full of blessing. But because this blessing came to them (the inhabitants), I might think that they would eat and be sated; it is, therefore, written (Ibid.) "and you (Israel) shall eat and be sated."
You will eat and be sated, and they will keep themselves, their sons, and their wives and daughters (from eating them) so that our forefathers come to the land and find everything full of blessing. This is as we have said (Ibid. 11:10) "from which you went forth" — When you were there, it was blessed for your sakes, not as things are now, there being no blessing in it, as when you were there.