Because they wish to crush the concrete ideas of the students. If a student even clings to an idea of emptiness, he betrays all Buddhas. Bodhidharma Mystics talk about emptiness. When they do, they are likely not well understood. The knowledge of emptiness is not attained through words.
Words can only point the way.
They say, look, there s this thing you need to understand, but it s not a thing. It s nothing really. Emptiness is the trough between waves. Emptiness is the writer s mind, not blocked, but waiting for an idea to present itself. Emptiness is the 0s in computer logic.