Why have I never seen a squirrel who died of natural causes?

I was watching the squirrels bounce from tree to tree in my back yard and I thought that looked like fun. Then I thought that it must suck when they get old. Then it occurred to me that in all my years alive, I have only seen road kill squirrels. Do they all commit suicide after a certain age or do other squirrels kill them and eat them or do they bury each other. LOL
Feel free to give other suggestions to solve this mystery. I know in the city I only seen road kill ones and the same for in the country-side squirrels.