The earth is 4.5 billion years old, and by most estimates has only 0.6 billion years left before the sun's light is insufficient to sustain photosynthesis. Even if some life survives that, earth will eventually burn up as the sun expands.
Some catastrophic event, human-caused nuclear warfare, asteroid, bioweapon could easily destroy all intelligent life on earth. If that happens it is very unlikely intelligent life will have time to evolve before the earth is destroyed.
Why gamble on an event like that not happening given how close we've come? The universe is vast, why don't humans have a duty to protect life and bring it to other planets asap? It's so strange that environmentalists want to preserve natural life but steadfastly oppose colonizing other planets (the only way to actually preserve it).