Why did everyone make Israel give back the land they won?

Israel won land in every war they have fought, and everytime they win, we make them give it back. If the arabs will try to kill innocent children, then why dont we just let Israel annex all the lands they took? This seems so stupid. People who dont like Israel say that its not their land, well guess what, the UN and UK made them a legitimate country, so they acted in self defense for all of their wars. To the victor go the spoils, thats how it always was. Thats how half of the arab nations gained their land, so why do we punish Israel for winning?