You are very right. We need to use every tool at our disposal over the next few decades if we are going to avoid a climate disaster, and afforestation plays a role in nearly all routes to avoiding that disaster [1].

But that leaves it in the biosphere, and new forest is vulnerable to rapid deforestation and conversion to CO2 (fires, cultural change, etc.). We would ideally shift some of that highly mobile bio-sphere sequestration to far less mobile geologic sequestration. Most likely with technology that we haven't yet invented.

[1] chapter 2, afforestation is part of AFOLU carbon dioxide removal (CDR) https://www.ipcc.ch/sr15/