...because they cannot use Ziplock bags. Let me illustrate.
Ok, basically the way they work for you non-baggy users is that there are two colors that the zipper is made out of. Generally they are yellow and blue, and see-through. This makes the zipper turn green when it is zipped (cause yellow and blue make green, yay). Now, these poor colorblind people will read the directions, but upon use have no clue if it is actually zipped or not! Now for your viewing pleasure some of the more amusing pictures that came up with I searched for "ziplock" on google:
That is all for now.