I see them over here all the time... you'll find them on jerky packets, for example, or as the initial seal on nearly any re-sealable bag (think of a bag of cereal with a zipper on top). The problem is that if the Seal God (blessed be his flippers) doesn't smile on you, you'll end up ripping off a sliver of the top of the bag and completely failing to get a decent opening. This has caused much wailing and gnashing of teeth. I think the material used for the bag/packaging is definitely a factor--I can pretty much always make a good tear on a ramen flavor packet, but I don't know that I've ever had a bag of jerky open properly.

God, all this talk of cereal and jerky and ramen makes it sound like I eat only crap. I eat vegetables, I swear!