Besides, as somebody already pointed out, there is that CO2 on the air that we actually want to get rid of. It's nothing compared to the rocks, and a little harder to get, but getting it first would improve things a lot.
There needs to be more appreciation for the laws of thermodynamics when discussing technology. Everything is not a 1-dimensional reduced abstraction.
For this reason I have long been slightly baffled that development of compostable/biodegradable bio-based plastics is such a priority in materials research. Sure, it's interesting in the very long run, but for the foreseeable future, converting atmospheric CO2 (via plants as an initial step) into a long lived, inert material that can just be buried after an initial use seems like a benefit.
In addition, things that biodegrade don't immediately just turn into CO2. Things like biomass (that is everything alive and dead that isn't decomposed) use a lot of that carbon. A significant fraction of the carbon in rotting stuff doesn't end up in the atmosphere for decades or longer. The carbon cycle isn't just "CO2 becomes plants which become CO2"... there's a lot more steps in between (for example, next time you eat... you are a direct next step!). Some of those steps take a very, very long time.