If there are a billion people in the world able to plant trees, that's a 1000 trees each. That's about 10-20 days of work per person (I have planted about 50 trees in half a day once). You're right, this would be easy.
You will need land which don't encroach on agriculture or other industrial use land. Land which gets enough rainfall every year. Also planting is just one step in the process , you will need to maintain it, if the trees actually have to grow.
tree planting ≠ forest planting.
But on that notion a “productive” artificial forest solves this issue.
build forest with plants that provide utility. But are family to the local environment. it’s a concept that has been proven