Carbon storage in nature is temporary and therefore is not equivalent to permanent fossil fuel emissions
This is the crux of it. We are burning C-based fuel sources that would otherwise never enter the C-cyle, so even if we temporarily tie up that C in plant biomass it is eventually released. Granted, it's taken up by other plants eventually, but we should view forests as buffering capacity in the C-cycle, not the sequestration capacity.
Think of it this way.
You have a bathtub. Attached to bathtub is a smaller bathtub.
You turn on the tap in the first tub, and let 'er buck. Eventually, the first tub will get full (where we are now). You're still ok for a little bit, since you have the second tub next to you (forests), but eventually the water is gonna run all over your floor.
Now you can keep buying tubs (planting) to improve your capacity to handle water, but that doesn't fix the problem. Meanwhile you have some jackass punching holes in your second tub, and other dipshits telling you that they will get you a tub to help eventually. They then fucking push back delivery of much needed tub. You also have people who don't think the tub is going to overflow, and people who don't fucking care if it does as long as their feet don't get wet.
So what do you?
Turn off the fucking tap, already. Christ.