ANSWER: "
Rashi asks your question in his commentary on the very first verse. In short, it is so that if the nations say 'You (Jews) stole the land,' we will be able to reply, 'The entire earth belongs to the Holy One, blessed be He; He created it and gave it to whomever He deemed proper. When He wished, He gave it to them, and when He wished, He took it away from them and gave it to us.' See
here for more."