Because of man's arrogance in the face of nature. "Oh look we can make heat and energy from this oil, it will never pollute a vast planet and we can never use enough of it to make it scarce" "Trash problem? What trash problem, dump it in the sea it will all dissolve in it's vastness" Centuries earlier Europe said we can cut these trees down and make energy there are too many of them for us to make a dent in them"
It usually doesn't end well and the OP is right we would literally be adding to the mass of the earth and we have no idea what effects that could cause.
Mining the other bodies in the solar system is just another cop out to developing sustainable lifestyles and only a delay. Trying to change the orbits of asteroids is a very bad idea.