I’m sure I’ll be easily shown to be wrong, but I assumed that it’s because the universe’s mass is finite and that the all-powerful infinity stones only freely manipulate the substance of the universe, and therefore work with limited material. In that case, doubling resources in one place means repurposing equivalent mass from other areas.

The other problem is how you implement this solution across the universe: do you re-engineering biological systems to be twice as efficient? Do you double the input energy? What about non-biological resources: do you speed up geologic processes? Do you simply increase the size and mass of each world and assume twice the resources are available and accessible? How deep of an understanding of each ecosystem is needed to account for all potential target populations, and can a single mind hold all that information in place while giving the infinity stones such a command?

Most pertinently, Thanos was a cynic. He didn’t trust any higher lifeform to not ultimately be shortsighted and selfish, and so his brute force solution was a culling.

Tbf, the system would be broken (in a functional sense) upon destruction. Cart before the horse with these statements, I suppose.