A very large amount of that fossil fuel was deposited during a period when plants could not be broken down successfully but microorganisms. In the current world much more of the carbon taken into a tree is released back into the atmosphere than presently.

That is: the original deposits were a process over millions of years, but the flow between stocks of carbon no longer behave as they used to. Whereas the stock of living and dead plants used to both remove carbon from the atmosphere, this no longer really the case.