The most basic answer is because of entropy. You eat, drink and breathe which requires chemical reactions to occur. These chemical reactions cause atoms and molecules to make new arrangements which requires energy. Over time errors occur where not every molecule is where it is supposed to be. DNA is information and information is subject to entropy.
Even if you externally did something to reverse the biological damage which happens over time in order to do that you require a template of an earlier state as reference. This means information has to be written, stored and retrieved. Again, information is subject to entropy. Even the information etched in the plates of the Pioneer probes which were designed to last billions of years in space will eventually break down. Magnetic tapes that haven't been played for decades still contain pops not originally on the tape. The Universe itself will eventually run down and for all purposes be dead.
Over enough time a biological entity will develop small errors in their genetic code. Any template that is used to revert the genetic information to an earlier state will also develop small errors in the recorded information over time. Gradually all small errors will begin to compound with each step into larger errors.
Everything dies. Since this is inescapable remember one thing above all else about death. Do not go into the light. The light is a trick.