system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do extremely tall people have higher mortality rates?
gpt
Because doorways aren't high enough for them and eventually they bump their heads one too many times. 

On a serious level though - this is a developmental defect. Organs don't scale the same way as body size does when it comes to supporting larger organisms, but humans don't have the necessary systems in place to make organs in the right way to properly allow for larger humans, so an exceptionally tall person is making do with organs that aren't quite efficient enough for their body's needs. Most importantly, gigantism is typically caused by overproduction of growth hormone. This makes  the heart thicker, but doesn't make it bigger, so it just has a really hard time moving enough oxygen and other blood around the body. Eventually, it'll have worked so hard that it fails, which is the most common form of death in gigantic people.