I trust glass and metal. Plastic, less so, especially in warm climates. But, when I used to smoke drugstore tobacco like Blender's Gold Golden Burley, I kept it in the bag it came in. Of course, those bags did not last terribly long.
Because those bags are gas-permeable. Like Jeff said--- if you can smell the stuff, that is why. They are shipped that way because it is cheap and easy. If you double up on the extra-heavy bags, that only slows the process. Glass and metal are the least permeable, so for long term storage . . .what is bad about storing pipe tobacco in a ziploc bag if that's how it came?