I think there are simply not that many uses for nails in emergency situations.
Using nails as intended:
- If you want to build shelter or other wooden constructions you would need a lot of nails - which are heavy.
- Hammering in nails with a rock is rather annoying - and you're not planning to bring a 500g hammer, are you?
- In any case, for shelter and tool use you can just as well rely on cordage (which should be in your emergency kit) - it is arguably more flexible in its use and can serve many different purposes.
- Nails are not easily re-usable, while cordage is. This is relevant, as re-usable means that you need to bring less of the thing which means a smaller and lighter survival kit.
Using nails in improvised ways:
- Yes, there are probably a hand full of uses for nails other than for nailing things down. Maybe improvise a fish hook or something... But frankly, nothing comes to mind that would be really useful and which cannot be done in a different, easier way with the other things your standard survival kit will already contain (such as cordage, tape, wire, ...).
- To work nails into shape you'd likely need pliers - and you probably don't want to carry those just for this, as they are heavy and don't serve too many purposes.
- Nails are primarily to nail things down, it's what they are good at. The kit you bring with you is not made from wood generally, and nailing mostly isn't really part of its construction or fixing any part of it. And while some of the resources you can likely rely on in a survival situation involve wood, nailing wood pieces together is simply not something that is needed too often.
So the answer really is: people don't put nails into survival kits because they don't solve any problem that can't be solved by other, easier/better/quicker/more flexible ways.
So what are better alternatives to nails?
- For fastening: use cordage or create wooden fasteners/dowels using your knife.
- For fish hooks: bring some actual fish hooks, they weigh essentially nothing.