Much, much stronger neck joint. The grain of the wood can run the entire length of the headstock and stay parallel to the direction of stress. And glue really is stronger than wood. Remember most guitars have a large truss rod route at that joint, that combined with short grain on one-piece necks is what leads to Gibson headstock breakage.
It also saves on material costs because they can buy/cut thinner neck blanks, the headstock is cut right from the neck...cut diagonally, rotate the headstock piece, and glue.
But remember, Gibson headstocks aren't one piece either. They have wings glued on for little more than shape.