It's just how systems work in a general sense, energy counters entropy, if you can put new energy into a system you can maintain order. Fundamentally bodies don't need to break down as there's nothing theoretically irreplaceable or irreversible, so long as there is still energy available to inject into the system.

We are biological machines that evolved to reproduce and pass on genes, and the genes that consistently resulted in survival with successful reproduction formed a machine with a lifespan as long as ours. Many other creatures live far longer or far shorter lives.