The key regulating signaling molecule is nitric oxide. At high NO levels the quality control systems are cranked up and a greater fraction of organism resources (ATP) are devoted to quality control. At low NO levels that ATP is diverted elsewhere. As systems deteriorate, they become less efficient, they produce more ROS, and NO levels go down to compensate. This accelerates the deterioration. Then you die.

The ultimate answer is this: Life is short, then you die.

The proximate answer is this: You die soon enough that life is short.

The phylogenetic answer is: Your ancestors had these problems so you have them too.

Interestingly, this model is one of the shortest lived organisms. They aged in the lab in a matter of days or weeks at a rate equal to years in humans, right? So it scales.

Now, we need a look at the same exact system in gobi fish and giant clams.