>

It's usually only handguns that take extra precautions against dropping since they're more likely to be carried and easier to drop. On semi-autos there's usually a plunger that has to be moved out of the way of the firing pin (which will only move if the trigger is pulled), so even if the inertia from the drop happens to force the firing pin forward it will just hit the plunger and not the primer of the round.
On modern revolvers, the hammer is flat faced, instead of having the firing pin on the hammer itself. The flat face can't actually hit the primer or even the firing pin directly. Instead it hits a transfer bar that then pushes on the firing pin which sets off the cartridge. This transfer bar is normally lowered, and is only raised when the trigger is pulled.
On long guns, the only thing holding back the bolt/firing pin is the sear, which is supposed to only release it when the trigger is pulled. But with enough of a jolt, and probably a worn sear, the firing pin/bolt can slide forward and set a round off. The safety on this long guns doesn't actually prevent the gun from being fired, it prevents the trigger from being pulled, most of the time, these are essentially the same thing , but obviously, not every time.
I don't see a technical reason why we couldn't add a similar mechanism to long guns, and knowing the gun world there are probably a few obscure models out there who have. I'm not a gun expert by any means but if I had to make a guess it's probably because it'll add more complexity and cost when it's rarely even an issue.
Way I've been taught is that long guns should never be kept or transported with one in the chamber and the safety should always be on. Personally I don't carry them loaded at all, because I only bring long guns out to have fun at the range, not to fend off a small army. Handguns are a different story since they're most likely used for self-defense, and a lot of firearms enthusiasts argue that you need the gun "cocked and locked" in case a time comes where you need to pull out a gun and use it immediately.
In order for this happen the gun had to be
1) Loaded
2) Chambered (and/or cocked depending on the type)
3) Safety Off
4) Left out in the OPEN in the BACK OF A TRUCK where a dog could step on the trigger/action while being ready to fire
5) potentially with the muzzle already pointing in the direction of the person 
This is so irresponsible it sounds almost impossible to believe. >

We had a dude in my hometown who rolled his pickup in a car accident (his fault, he ran a red light). This asshole had a shotgun just sitting loose on his backseat that went flying around the cabin when it rolled and discharged because he was a dumbass and had it loaded and chambered with the safety off. Thankfully, no one was killed or injured, but the shot hit a nearby residence with people inside of it, which made the charges against him far more severe. >


because he was a dumbass and had it loaded and chambered with the safety off

It's possible the safety was on, but this is why you're not supposed to keep a shell in the chamber of a shotgun. It doesn't require a trigger pull for it to discharge. Most pump shotguns have firing pins that can impact the shell without the trigger's involvement. >

Wait, really?  Why?