You could say the same thing about ingredient lists or expiry dates on food items, they only show what should be in there, but still someone might have poisoned your cookies. It's reasonable to assume that there are no real lies on the package, that your cookies will contain exactly and only those things listed in the ingredients and that they didn't expire five years ago. This doesn't prevent the cookie company from telling any lies (companies being called to court for false advertising is not uncommon), but it creates some trust that otherwise wouldn't have been there. I don't extensively read the ingredients on my cookies, but knowing they're there sufficiently satisfies my curiosity (side note: I'm thinking interpassivity may be at play here) and I don't think I would have bought them if there were no ingredients listed at all. Of course, this in itself doesn't explain why I would eat anything at a restaurant or at a friend's place, because usually the ingredients of the food aren't listed there.