There is only so much ground that can be covered with trees, and it stops those areas being used for other purposes.
There are no practical limits for how much CO2 you can store underground!
There are no practical limits for how much CO2 you can store underground!
Energy cost is the practical limit.
With the full potential unlocked, we'd be able to plant so many trees that they'd capture 2x or more of the total CO2 emissions (total emissions, not just the airborne excess) which means we'd start capturing the past excess emissions as well.
Since land is finite and the number of new years is not, that can only be true for a finite number of years.
Also, we need to capture more CO2 than is emitted to get down to natural levels.
CO2 stored underground in the form of plants dying and rotting for the benefit and use of other plants is also "carbon capture".