Why are humans steward to your finite earth?

We like to think that the world is ours. Since we are the most powerful species, we can make that claim. And if the world is ours, then it is our responsibility to take care of it, and if we don't take care of it, we will have only ourselves to blame when it gets messed up. If we ruin the planet, we do not really have the option of moving to another planet. Mars is an extremely barren, airless wasteland, and even so, it is much easier to live there than any of the other planets to which we might hypothetically be imagined to move, as an alternative to Earth. The solar system is not filled with prime real estate. The Earth is pretty much all we have, that is habitable. The Earth and the international space station, where possibly half a dozen people can live, if the station is regularly resupplied from Earth.