Assume it was feasible to create a breathable atmosphere on Mars - that you could just melt the polar ice like in Total Recall, and boom, instant blue skies. Ignore the radiation Mars has been bathed in over the aeons, the toxicity of the soil, the low light levels which might make photosynthesis (and, thus, an ecosystem) impossible, and the lack of a magnetic field to shield the planet from solar winds[0]. How would you keep the necessary amount of atmosphere on the planet given its small size and gravity relative to Earth?

[0]https://en.wikipedia.org/wiki/Terraforming_of_Mars#Challenge...

So, it's not that it's bad, it's that it's a hard problem.