I can't speak for everyone, but for me it's because that efficient strategy has too little to do with actual survival. If the trader is OP, instead of having a zombie survival game, you end up with more of an apocalypse mogul game. This happens with a lot of space games. You want struggle against alien factions, huge space battles, sector domination, necessary exploration for exotic resources, but then you realize that buying unobtainium at one space station and selling it at another is all you have to do.