S.
I believe it is because in the olden days veggies were sold in cans that contained lead, so storing them in the can exposed you to lead poisoning. Also there was a scare in the 70's about a bacterial poisoning through canned goods. All in all, even if we can't think of a reason, it is just really not a good idea. I fight with my husband about this issue, too, and it's very surprising because in all other respects, he is a germ-a-phobic that drives me crazy.