Actually, if the forests were large enough, they could do that.
The problem is two things. You have to remember that the area annually burned is measured in one to three.... It went from one million hectares to about three million hectares on average at the present. Recall that the forest area of Canada is about 347 million hectares. When 1% of the area burns, 99% of the forests are in various stages of regrowth. I'm ignoring harvesting for a moment here.
We have to understand that the forest fires are affecting a small proportion of the landscape, an increasing proportion and causing important emissions, but a significant part of these emissions is removed by the other 99% of the forest area that did not burn in this year.