Why doesn't patients with chronic acne's body lower sebum production itself?
Short answer there is no feedback mechanism to tell the body it is overproducing, inflammation can have a huge number of causes, many of which could actually be worsened by decreased sebum production. 

Also "overproduction" is a misleading term; it is overproduction compared to the norm for the population and may be perfectly normal for that individual. It is like asking why redheads don't start growing black hair when surrounded by surrounded by people with black hair.