Even if all of the wood rotted immediately when the tree dies, that would still tie up carbon in the biosphere. But it’s slow and results in increased carbon in soil, litter, etc. It’s not like we’re lacking tools to account for this:
https://www.fia.fs.fed.us/library/database-documentation/current/ver80/FIADB%20User%20Guide%20P2_8-0.pdf
But you can’t grow trees everywhere. These folks estimate the increased carbon sequestration capacity of CONUS forestlands to be <200 MMT CO2 per year:
https://www.pnas.org/content/117/40/24649#ref-15
To put that in context, US emissions in 2019 were ~6 GT CO2 eq.