Trees aren’t completely ineffective at carbon capture, they just need to be used properly and with knowledge of their limitations: much of their carbon will be released rapidly in the event of a fire (around 20% of a tree’s mass is underground, which is why I don’t say all), the carbon they captured will be released over a course of years after they die, and carbon capture rates aren’t static and will sharply decline at a certain point. To keep such a project going requires continual replanting, much of which can be accomplished by the trees themselves if the project is done properly.
Most tree based carbon credits are pure scams and were always intended to cheat the system. Done honestly, we could still use trees for carbon credit but you’d need a LOT more trees per unit credit (compared to current scheme rates if they were even honest) and the project would have to estimate likely potential carbon losses and bake those into the valuation.