My guess would be a need to create complex molecules for other building block types. Unless they can find the lost coal pits or oil wells, Mars will have 0 hydrocarbons. Don't you need complex molecules to be heavy enough to stick around? CO2 is light enough to not be held by the weak gravity maybe?

Why? Greenhouse gasses are essential for terraforming Mars. What we deem industrial pollution woyls help contain/retain an atmosphere. Further, trees and other oxygen generating foliage consume co2.

The concept of making fuel out of the CO2 in the martian atmosphere is so common in sci-fi I actually thought a process already existed to do it. Maybe there's a theoretical reaction but not process to realize it with equipment that would fit on a space ship.

The long-winded way of saying they're seeking CO2 reaction processes is funny.