Sure, on an absolute basis, someone who started with more than me will end up with more. But why does that matter to me? I only have my current resources to consider.

We all have some resources at our disposal. Understanding that slowly growing them over time can really add up is helpful regardless of where you start. In fact, I'd argue that knowledge of compounding is more helpful for those with little. Wealthy people aren't in need of more wealth, so the concept is less useful for them.