This is a good point, because trees do absorb a lot of CO₂ from the atmosphere and have been doing so since long before mammals appeared. However, humanity is constantly cutting down trees, both legally and illegally. Also, due to the heat and arid climate, many young trees die before they reach their peak CO₂ capture capacity. Therefore, relying on tree planting alone to reduce the planet's heating in the long term is a bit risky.
Moreover, in the current consumer economy and with the number of polluting companies continuing to emit greenhouse gases (i.e. those that cause the so-called ‘greenhouse effect’, including CO₂) into the atmosphere, our planet needs all possible methods, including planting and caring for trees (natural DAC) and using man-made technologies that absorb carbon dioxide (engineered DAC).
However, reforestation is still a critical issue for the environment. If done right, reforestation reduces soil erosion and increases biodiversity, while additionally absorbing carbon dioxide from the surrounding air.