[L_226]

Because trees cannot be grown just anywhere, they require specific [land, water, nutrient] conditions that also happen to be the same things that are already under pressure from our civilisation. This is caused by things like extensive monocrop agriculture, grazing etc. Furthermore, trees do not necessarily sequester carbon beyond their lifespan - the sequestration is an additional process that may or may not occur naturally. Think of old trees dying, falling down and decomposing vs old trees dying and being covered by mud.