Because clearing forests for agriculture INCREASES the effect of greenhouse gasses and reduces the available number of young, fast-growing trees that (temporarily) remove CO2 from the air and (temporarily) store it in the wood.
ONLY if you HARVEST (cut down) the young, fast-growing) trees, and PREVENT old-growth, slowing growing, stagnant forests from remaining in place, can you use trees to reduce CO2’s “disastrous” impact on the environment.
Otherwise, more CO2 (and fewer croplands) simply means more and faster-growing trees.
However, people must starve to death because they can’t eat meat, and can’t grow crops, and can’t get energy to protect themselves from winter’s cold and summer’s heat.
Which is actually the AGW extremist’s goal in the first place.
The first thing that must be done is dispel this notion that CO2 causes a rise in temperature. Numerous studies show it doesn't. In fact, it's been show to be exactly the opposite.