A penny or two doubles the price of the bag. It also creates an easy way for the consumer to seal the bag and therefore will lead some people to eat less. Both of those things are bad for the manufacturer, so why would they do it?
Because, if they're the first manufacturer to do it, then people who "I hate when my potato chips go stale, I always throw out half the bag" will flock to that brand.
It doesn't matter if people eat less - it's how many they purchase. At least, I hope, their target market isn't people who make a special trip to the store 7 nights a week for potato chips.
Of course, then the other brands will switch their bags over so that they can compete. And, if many people are like me, they only buy whichever brand is on sale for BOGO; I can't remember the last time I paid full price. And, if the brand on sale is a crappy brand, I just don't buy chips.