I would say there are a few answers to this question. 1. First, not all spirits are here because of tragedy. It is only one possibility. Of course according to your example that would just add more spirits, haha. 2. Second point is probably the most important to your question. That is not everyone who experiences a tragic death would stick around. Only a few. What determines this? Well heck who knows. That would depend on your personal, cultural, or religious belief system. 3. My Third point would be that in cases such as mass tragedy the place the tragedy occurred can become overwhelmed with a negative energy field. It doesn’t necessarily mean all the souls/spirits are trapped there but it’s just the resonant energy of that tragedy that has remained at that location. Some believe that such strong energy can cause odd happenings making people believe a spirit is there. 4. Even if all the tragedy victims were spirits/ghosts it does not mean we would all have the ability to see them or detect their presences. So I suppose it could always be a possibility. 5. Oh I just thought of another I’ve heard. And that is the possibility that these spirits are on a different plane of existence than us. Only sometimes does the veil reveal this existence allowing us to see the spirits. So billions could live in this different existence and we would only be privy to it whenever the veil is so lifted to reveal whatever it reveals. Of course it all depends on your belief system wherever it comes from. But this is some information I have picked up about the topic.