The point of planting trees in Denmark is not to cut CO2 emissions. The point is to restore biodiversity and the health of the environment. I assume the situation is similar in countries like the Netherlands.

Climate and environment are two separate things, and are in fact sometimes at odds with each other. Denmark is doing semi-alright on climate, but is absolutely terrible on environment. Aquatic ecosystems in the country are basically completely destroyed by agriculture, to the point where previously productive shallow waters are completely dead due to oxygen depletion.