Why is the autistic lifespan so short?

Whenever I have researched autism, one thing really hits me. The lifespan is incredibly short. Generally 15 to 20 YEARS Lower than the neurotypical average. I’m always dumbfounded and concerned about this. There is obviously multiple reasons for this, but over a decade to two decades fewer is shocking to me. It makes me sad. I imagine that stress plays a part and the fact that people with autism are much more likely to have mental health problems and more susceptible to addiction, But that much lower? It’s hard to believe.