because garlic and peppers do not make an effective pesticide.
in all seriousness, its a subject ive been super curious about. ive always wondered why preppers(i personally know several) always follow the organic model of agriculture, when modern agronomy has so much more to offer in terms of plant productivity. if one is willing to stockpile like 125lbs of ammo and silver coins i dont understand why they wouldn't stockpile fertilizer and pesticides as well.
200lbs of calcium nitrate will fertilize an acre of land for one year. and a few gallons of cheap permethrin/malathion concentrate will kill any bug trying to eat your plants. in the face of starvation i would think its logical to put aside ones preference for organic foods in favor of a far more reliable and productive means of agriculture.