There's a food industry term called 'active water' which represents the moisture content of a food. High active water foods can grow bacteria and mold very easily as the microbes have a near ideal wet environment in which to grow. Low active water foods do not bring enough moisture to have microbes thrive and the food desiccates rather than spoils when left out. This is why McDonald's hamburgers don't grow mold when left out. They simply have too little water content to support microbial life and turn into mummified rocks instead. As for store bought bread, it is generally low active water but storage conditions matter. Bread is hydrophilic and will draw humidity from the air so high humidity environments will lead to rapid mold growth. Additionally, the mold spores originate at the food processing facility so your bread is already contaminated from the start. I would suspect that store bought bread isn't molding because the adulterated formulae that are industrial bread products have created less ideal conditions and differences in pH and other factors end up inhibiting mold growth. Sure there could be nefarious acts behind this, but without actual research and analysis your observations are merely anecdotal just as my speculative probable causes are. My mother-in-law continues to buy store bought bread and I have thrown out many a loaf in recent months because she often forgets to properly close the bag after use. All of the types of bread she buys are molding so my experience is different from yours but doesn't prove or negate anything. Sample size of two is just bad statistical rigor. More work needs to be done to determine what may be going on here.