It is likely for the same reason beer never lasts around here once you open the case.
He he he.

I have had ammo sitting around for years. Some of the casings were even turning green. It still went bang and grouped well on the range.

I have also read stuff from manufacturers that said ammo would go bad and not shoot well after a year in storage, and that it didn't matter how it was stored.

Ammo sweats?
If you mean condensation my rifle shells used to do that every time I came into the house after hunting in the winter.
Just empty the magazine out before you go inside, hang the shell belt up, and the frost that condensed on them soon dries off.

It was the condensation on the rifle and scope that used to worry me more.