Because the world is not constant.
If we had a completely stable, unchanging planet, then life could exist in an optimal, unchanging (unevolving) stasis. There might be slight changes in organisms, but eventually even those would stop as the populations of all organisms would become more and more homogeneous as they reached their optimal state of development.
But the world is NOT constant. Climates change. Sunspots change radiation. Meteors hit the earth. Comets pass close and dump space dust. Volcanoes erupt. Oxygen levels fluctuate. New toxins appear. New diseases or predators or food sources move into different environments, and old diseases or predators or food sources die out.
When this happens, any species without the ability to evolve (which just means change over time) goes extinct.
Without evolution on a changing planet *ALL* species would go extinct.
But now the question is ... is a constant planet even possible once life exists? Evolution *causes* evolution! The reason an environment changes for an organism is that *other* organisms are changing ... they are spreading, migrating, populating, dying out, and themselves *changing* (evolving).
So it may be that once life exists on a planet ... even a completely stable planet with no seasons, no tides, no meteor strikes, etc. ... evolution is not only "good" in the sense of avoiding extinctions ... evolution is *unavoidable*