The nick you're describing creates what structural engineers call a 'stress riser' in the material of the bag. It focuses the force you exert on the bag to a small point, from which the tear will originate.
Stress risers are a common cause of structural failure, and are usually designed around. Some well known examples where they cause failures: rectangular window openings in aluminum fuselages, knots in fishing line, a poorly designed head tube lug on a steel bicycle frame.
In this case, however, the failure of the material results in a good outcome: snack time.
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!
They frustratingly don't often work with greasy fingers. Which you might have if said sachet or packet comes with, say, a fast food meal, or from inside a bag of crisps (like Salt n Shake in the UK).
Given the shape of a bag, it seems pretty difficult to put shear force on the bag. Since the sealed layer deforms and crumples, if for example you grabbed onto both sides of the middle of the top edge and tried shearing them apart, they would just reorient so that the forces are now more or less normal to the glue...
Apply thumb and forefinger on either side of the seal. Press together and slide. Coefficient of friction between finger and seal might need to be increased somehow (perhaps by texturing the material differently at that point).
It's still sealed, but the nick gives you something to leverage, so you can tear the packet open. Perhaps this isn't as common in the US?