Neither do health packs. Why would you find a health pack in an underground cave, or in the middle of a jungle, or on top of a desk in a lab, for example? The concept of them doesn't make sense either.
But, from a gameplay perspective, regenerative health works. It gives the user a prompt that you're close to being killed. So, you either find cover and time to recover or you die. Imagine how frustrating it would be if you've just made it through a really difficult section and die because you couldn't find a health pack around (there's none around). It makes the game less frustrating so that you can concentrate on the more fun things, such as exploring, adventuring, gunplay, uncovering more of the story, etc.