Because of short term thinking. Nobody cares about what will happen beyond the next election cycle, the next quarter, the next iPhone release. A bit more long-term thinking would not hurt anybody.
It's not about planning out 10,000 years, or even 100; it's something more abstract than that. What will your legacy be? Your family's legacy? Your country's? Humanity's? I don't have any personal connection to somebody who will be alive 1000 years from now, or even 200, but I still don't want to leave them a dead, depleted planet.
Is the clock a bit frivolous? Sure. But it's not about the clock. The clock is merely a symbol, that we have considered our place in history, that we recognize we are part of something larger, and we want to be good stewards of our future.