Because for all his talk about conserving resources and bringing balance, he's a mad man. He's a destroyer, not a creator. At best, he truly believes what he says and is obsessed with bringing balance because he lost his home because they weren't willing to make the hard choice. At worst, he's a genocidal lunatic rationalizing his desires. But going even further, Stark had him in a one-on-one conversation. So did Strange. Why didn't either of them suggest that to him? My thought: from the first beat, they only recognize these stones as weapons. Everything Thor told the Avengers about the Infinity Stones, and then everything Strange told Stark, was that these were infinitely powerful items capable of unimaginable destruction. If you're a cave man with no functional knowledge other than what you've observed first-hand and all you know of fire is that it destroys everything, you never think of using it in a controlled manner to cook your food. Nobody really knows much about these stones other than myth, including Thanos. "Doubling the universe and it's resources" just isn't the way these people think.