But that means that planting a tree binds a finite "tree lifetime" amount of carbon. So, in order to neutralize the worldwide carbon emissions using trees, we would need to keep planting them as long as we keep releasing CO2. So we would need to consume X square miles of agriculturally suitable land per year to simply keep the net emission at zero.

Now, IIRC, the energy density of oil is much higher than the one of lumber. I haven't done the calculation, but we might simply run out of suitable area way before any reasonable effect is reached. Don't forget that the logistics of tree planting will spend some CO2 as well, and given the finite CO2-per-planted-tree amount, it could be considerable.

Also if the trees have not spread naturally in the past millions of years in the area where you would want to plant them, there might be some ecological reasons to it. Hence, they may not survive long-term, or may get wiped by forest fires, or may require irrigation, fertilizers or other support that also produces CO2.