Because reducing each human's consumption by X% is always going to be less effective than flattening the exponential curve of population growth. It's not even close. Especially when the global trend is towards higher per capita consumption rather than the reverse. Reusing grocery bags in the US (hey I do it too) is nothing compared to the mass modernization going on in huge areas of the world.

And "culling" (I assume you mean through violent means) isn't necessary. Simply reducing birth rates achieves the same solution.