Because it forces us to recognize that the risk isn't to "the Earth" as a separate entity, but to ourselves specifically (well, our future descendants).
We've already confirmed through our behavior that humans largely don't care about any other part of the Earth ecosystem, so asking people to basically do the right thing as a favor to the Earth are probably wasting their time.
I swear, if one more person quotes an old George Carlin bit, as if it were still clever or useful...or as if said person didn't know what was meant by the statement.
