Why was the average life expectancy in the 17th-century chesapeake so low?

That's because there were a lot of diseases and illnesses that could kill people that weren't treatable at the time like typhoid or dysentery. There wasn't even enough women for all men so the men killed each other over who gets to be with what woman. They also had problems with the natives because of spreading tobacco plantations and they could also get killed by animals. Most of the people wouldn't make it to their 50s.