Answer
Because love seeks repetition.
A devotee does not chant merely to extract benefits from Krishna but to increase love for Him.
When people truly love one another, they do not think, "I remembered you once; that is enough." Love naturally wants continuous remembrance and association.
Similarly, repeated chanting trains the heart to love Krishna more deeply. The aspiration is to reach a state where one can chant continuously, from morning until night, simply out of love.