I'm just guessing here , but my take is that boughten oir custom bags were more likely to be lined . For me , the lineing is just a prroblem , sooner or later you'll catch a hole in the lining and it'll keep getting in the way , Lining the flap as mentioned can have a practical purpose . The lining in the bag also would tend to take much longer to dry out if caught in heavy rain or a dunking in crossing water . I know lining can make a pretty bag , but for real field use I like mine without it .