I think that is simply because there is no room and as important as shelter is, the theory is that if your kit is too big then you leave it behind and then you end up with no gear at all.

This is true and I would not dare say that having some type of a sheet of plastic isnt VERY useful but then you must also consider kit size. Notice that even Cody carries no shelter in his smallest of kits (the one based on the knife sheath which is the one kit he says he is never without).

I know I for one am very guilty of making big all-inclusive kits and then leaving them in the truck when I venture out on foot. My large truck kit (designed to support 3 people) contains a vynil tube tent, and Storm Shelter Pocket Tent and several space blankets, bivy sacks and ponchos, but none of this ever goes with me when I venture out on foot. Improvising shelter is not too difficult if youre in a decent physical and mental state when it is needed. Of course when youre cold, wet, scared, have a broken arm, and are running out of time before freezing to death, a large sheet of plastic probably makes all the difference in the world. But still one must consider the fact that it's all useless if you wont carry it.

I day hike several times per week and backpack overnight (1 or 2 night trips) about twice per month. I often go off-trail so the potential to get lost is pretty good if I'm not careful. Lately what I have been carrying is a small PSK which is actually part of my normal EDC regardless of where I am (home, office, out in the woods, etc) and a knife (also part of my normal EDC). The only thing that really changes is that when I go on a hike I usually (though not always) swap out my 3.5" EDC folder for a 4.5" fixed blade knife. Of course it is still pretty warm down here in TX and I'm hiking without a jacket too. Heck this week we have had highs in the 90s. Granted, in February when it's colder and I'm wearing a heavy down coat then you will be MUCH more likely to find me carrying a Storm Shelter Pocket Tube Tent and probably a space blanket also.