True. However, one way to stop the salad from wilting is to take it out of the bag when you buy it or the next day. Line a container with paper towels, put the salad in and seal it. Shake it up so the moisture is absorbed by the paper towels. You can keep it fresh for another two days.
Funny, I buy bagged salad (3 at a time) and they last fine for 6-7 days - opened and unopened. However, when I buy cookies, they don't last for more than 24 hours.