That’s the obvious reason for lack of trees—lack of water. The Sahel region is marginal—not as dry as the Sahara so it gets some rain except when there is a drought. So it may be possible to plant drought-resistant trees which can survive periodic droughts. But why hasn’t Mother Nature already seen to that if such plants are available? Maybe the locals burned them all for firewood.
Even if drought resistant trees were grown and there was enough rain for them to survive, those kind of trees grow very slowly. Would take decades if not centuries to make any significant difference. Not very practical. Doomed for failure from the start.