Infinity War did a pretty good job explaining Thanos’s motivation to give you a picture of why he’s doing what he’s doing. However, most people probably agree that even with his motivation of limited resources and the universe’s population’s potential to expand past them, killing half the universe is pretty extreme. Leading to the question: if he can wipe out half the universe with the Infinity Gauntlet, why not expand the resources?
There are a number of ways to tackle this question, but to start, let’s look at what Josh Brolin himself said. In an interview with Stephen Colbert Brolin told him that Thanos didn’t double the resources in the universe because it didn’t even occur to him that that was an option. He describes Thanos as being too callus to approach the situation anyway than he did. Basically, what Brolin is getting at here is that Thanos just doesn’t care about the people of the universe, he seems to only care about the universe itself and its survival as it relates to his own goals. What that seems to mean is that Thanos views living creatures as necessary to the universe’s overall survival, just not the amount that it had, so he views killing them as the way to obtain the balance he seeks. His problem after all wasn’t that life existed, it’s that too much life existed, so if he made more resources, he’d probably think there were too many resources now. Think of it as Thanos’s cosmic OCD.
The next option comes from the idea of limited resources itself. If the universe itself had some kind of limit on the resources it produces, then the universe itself must have some kind of limit, it’s not infinite in this scenario. So, in that case if Thanos could make it so that there will always be enough resources for everyone, the resources and living beings might overflow the allotted space they have. If he only doubles the resources, then that runs the risk of living beings outgrowing them again and it doesn’t actually solve the problem.
On that same note, if people always have all the resources they’ll ever need, they run the risk of becoming weak and complacent. Thanos’s goal is to allow people the chance to survive, not to coddle them or guarantee it. Someone as pragmatic as Thanos wouldn’t want a universe like that.
One final option is that Thanos just was not physically able to make enough resources to maintain the current population growth. Keep in mind that just getting rid of half of the life in the universe almost destroyed the gauntlet and seemed to send Thanos into a near death state. That was just one action, a very large scale on sure, but once he finished it he wouldn’t have to do it again anytime soon. As stated before, if he just doubled the resources, there’s a chance that the population would outgrow it again. So if he wanted to maintain a sustainable level of resources it would take a constant action to replenish them, which would cause a constant strain on Thanos and the Gauntlet. So Thanos may have chosen the option to wipe out half the universe because it was the best option he and the gauntlet could handle. Plus, it would likely take more time to replenish half the population than it would to go through double the resources.
Or maybe it was mixture of all of these options, but at the very least you could reason out why Thanos went to the lengths he did.