There is limited surface area on the earth where trees will grow. If you want to utilize that maximally to sequester carbon, then you need to cut down mostly grown trees (which have stopped sequestering carbon) to make room for new trees (which can sequester lots of carbon). Then, in order to prevent the carbon contained in the trees you cut down from re-entering the atmosphere, you need to use the wood in some way where it does not decompose or get burned (to build houses, basically).