Because if we're the only one's actually capable of appreciating the beauty of the universe, if we are in actuality the only mindful expression of the universe, we are more incomparably valuable than any other element existing as part of that universe, and an irretrievable loss once gone.

You're asking for the meaning of life there. For some it's pure unadulterated hedonism, for others it's religion, for others it's science or technology, for others it's their children, and so forth and so on. But for many the continuation of our species, whether or not we're a part of it, is certainly quite a crucial part of it.

As an aside from this, you also don't know your statement to be true. Our planet has, multiple times, killed just about every single living species on it. We'd probably stand a decent chance of partial survival due to technology and intelligence, but on the other hand we might not. Think of all the [relatively] near miss large asteroids that we detect only long after it would have been much too late to do anything. And that's something we can actually control. An unfortunate gamma ray burst or other form of harmful electromagnetic radiation could be GG earth with little to no warning or even chance to react.

The chance of an extinction level event happening at any given point is very low of course, but they have happened and will continue to happen. It's just like winning a lottery, one that you'd very much prefer not to win.

Because it keeps us from slipping into the endless reducible hole of nihilism.

Because not all of us are so selfishly narcissistic and are rather able to consider the effect upon the lives, extant and not yet born, of our current decisions. Your apparent lack of empathy, and similarly our cultural worship of selfishness and self-deception, are foul clues as to our eventual doom. If choices today are critical in determining the viability of human life over the next several decades, but those choices conflict with the selfish wants of children in the present, and those present-day children choose to disregard future generations, then what hope do we have?

The Great Filter for our species is capitalism.