Why Doesn't the Buddha Go to the Formless Realm to Teach the Dharma

The devas of the formless realm have no bodies, yet they have consciousness; where there is consciousness, there is the aggregate of consciousness (vijñāna). Although they have consciousness, they abide in meditative concentration and cannot hear the Buddha's Dharma, let alone contemplate it. Having no body, they are not humans among humans, but they are beings of the heavenly realm—advanced forms of life. Feeling, perception, and formations—the five universally present mental factors (sarvatraga)—are possessed by all eight consciousnesses. The devas of the formless realm have the feeling, perception, and formations of the mind consciousness, as well as the feeling, perception, and formations of the manas, and also the feeling, perception, and formations of the eighth consciousness.

The Buddha could teach the Dharma to the devas in the formless realm, but the devas of the form realm could not hear it, humans in the human world could not hear it, and ghosts and spirits could not hear it. The devas of the formless realm are so few, and they are all non-Buddhists who remain in meditative absorption all day long, so there is fundamentally no need to teach them the Dharma. From another perspective, one can no longer hear the Buddha's Dharma even in the concentration of the second dhyāna; how much less in the concentrations of the fourth dhyāna and above, where one certainly cannot hear or contemplate the Dharma. Therefore the Buddha does not go to the formless realm to teach the Dharma. The Buddha taught the Avataṃsaka Sūtra and the Kṣitigarbha Sūtra in the desire realm heaven and the form realm heaven, and then they circulated to the human world. All other sūtras were taught in the human world, so that devas could descend to hear the Dharma, and ghosts, spirits, and animals could also hear the Dharma; in this way, far more sentient beings could be liberated.