I've had tires stored for much longer without any signs of deterioration that were not even in a wrapper or container. So each of our experiences is anecdotal and circumstantial. I'll go along with those that said your environment where stored wasn't good. Ozone, fumes from certain chemicals and etc.
I'd recommend you only keep one or two spares handy and maybe don't buy them till your current tire is near it's end of life. If you stockpile tires, then you'll miss out on getting to try new and different tires as most likely you'll only stockpile the tire you are certain of.