The gigantic robotic super weapon had to come from somewhere (which the movie never acknowledges). It's likely that those who sent the Iron Giant to earth are not going to just give up after one failed attempt. This deleted scene shows the Iron Giant's memory of him and an army of others of his kind destroying another planet. The fact that the creators were so willing to sacrifice an army of them also strongly hints that there are plenty more of them, too. Also, the end of the movie explicitly shows us that the robots can survive nuclear blasts. And who's to say that he's still friendly after reassembling himself?