Because you didn't.
Here is what I think.
Some years ago as a boy scout group, we took off to St. Joseph, MO - north of Joplin a few hundred miles, to help out with some cleaning up from flooding along the Missouri River. I recall, though details are slim, that there were a couple of times that as we were out helping to clean up, many of us were getting dehydrated. Not to the point of something way serious, but I remember our scout leader leaving us to find some water because he was concerned. It wasn't an easy task to find it.
As simple as it seems, water in an emergency zone, is a most crucial supply. For those that have been affected as well as those that came to help. If the services and distribution system aren't working correctly, emergency supplies stocked up by those who were not affected in the disaster zone could be priceless. In Joplin, the Tornado took out a Walmart - and who knows what other retailers. Even if it was just a Walmart now gone, distribution of food and water in the area has become that much more tight.
Even for those residents whose homes are not gone, they would likely need an emergency supply of some items as they would not have access to normal supplies and supply lines.
But I think preparation isn't just for you. Honestly, I don't think it is so self serving. Perhaps partly. But mostly selfless-serving.
If you had emergency supplies, you would be able to go help, indeed, you would feel compelled to help, and give to those in need. If you were the one in need, you would hope that somebody would come and help you in your hour of need.
There are angels among us. And usually they are the ones that have taken the time to be prepared.