alt Hacker News250 years ago, you could make this exact argument about the British colonization of Australia, and it would be entirely correct. The early colony was a pure fiscal drain on Britain with almost no return.
Yet today it's the 13th largest economy on earth.
Think on a longer time scale.
We also have a lot of easily accessible resources via agriculture and mining, things Mars does not have. And even if it did having mining potential, the cost of returning the goods to Earth would be wild.