The organic matter that was once a forest has to go somewhere. If you put it in a big plastic bag and sterilize it that'd be CO2 neutral, but if you leave it to decay or even burn it you release most of the carbon in the plants as CO2

It's a one-time process. But so is forests absorbing CO2 as they grow bigger and denser, and we commonly consider that a carbon sink. Makes sense to do the same for the reverse