The way I look at a "survival kit", of whatever size, is to allow me to improvise and improve my situation should I become lost or stranded. As such, shelter is a given, since it's easy to fashion a brushwood shelter with a good, stout knife, a bit of cordage, and training. It's much harder to improvise matches, fishing hooks, medicines and other small, man-made items that may prove indespensible in a true survival situation.
Aslo, one only has so much space in a knife handle, or a pouch attached to your sheath, or whatever. So it makes the most sense to fill that small space with gear that can't be readily acquired from one's surroundings. I do, however, carry a dispoable poncho in my left cargo pocket. So, even if all I had was a little bit of line from my kit, and two trees, I could still set up a modest shelter. But I would still need fire, and while some may downplay the need for food, and others may put water further down the list that I, it's still too easy to carry each to ignore these basics. But, to each their own.
How much "shelter" can one cram into a knife handle, anyway? Maybe half of a Mylar sheet?