A self-sustaining colony on Mars will require many hundreds of billions (or perhaps even trillions of dollars) of dollars to set up. Imagine all the things you need to survive on Mars. Imagine all the infrastructure to build those those things. Imagine all the infrastructure needed to build that infrastructure, etc. etc. To be truly self-sustaining (i.e able to survive indefinitely without further input from Earth), a substantial portion of that infrastructure will either need to be shipped to Mars or built on-site.
Heck, you may as well ask, “Why don’t we have self-sustaining colonies on Antarctica?” Surely Antarctica is an easier terraforming/colonization project than Mars.
In one word: economics.
A self-sustaining colony on Mars will require many hundreds of billions (or perhaps even trillions of dollars) of dollars to set up. Imagine all the things you need to survive on Mars. Imagine all the infrastructure to build those those things. Imagine all the infrastructure needed to build that infrastructure, etc. etc. To be truly self-sustaining (i.e able to survive indefinitely without further input from Earth), a substantial portion of that infrastructure will either need to be shipped to Mars or built on-site.
Heck, you may as well ask, “Why don’t we have self-sustaining colonies on Antarctica?” Surely Antarctica is an easier terraforming/colonization project than Mars.