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

Beings in the formless realm have no bodies, yet they have consciousness; where there is consciousness, there is the aggregate of consciousness. Although they have consciousness, they cannot hear the Buddha's teachings, let alone contemplate them. Having no body, they are not humans among humans, yet they are beings of the heavenly realm—advanced forms of life. Feeling, perception, and volitional formations—the five universally present mental factors—are present in all eight consciousnesses; the formless realm beings also have the feeling, perception, and volitional formations of the mind consciousness, as well as those of the manas, and also those of the eighth consciousness.

The Buddha could teach the Dharma to the beings in the formless realm, but the beings 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 beings of the formless realm are so few, and they are all non-Buddhists who enter meditative concentration all day long, so there is simply no need to teach them the Dharma. On the other hand, within the concentration of the second dhyāna one can no longer hear the Buddha's teachings, let alone within concentrations above the fourth dhyāna, where one even less can hear and contemplate the Dharma. Therefore the Buddha does not go to the formless realm to teach the Dharma.

One can enter the concentration of the second dhyāna in an instant, and then knows nothing at all; exiting the concentration also takes just an instant, and only after coming out does one realize that one had just been in concentration, feeling that the body is extremely comfortable, the mind especially joyful, and the head particularly clear. A faint sound is enough to awaken the ear consciousness, and thus one exits concentration. Because the manas still has matters it is concerned about, when the time comes, if someone reminds one of an important matter—even if it is only the sound of a mosquito—one can hear it, and thus exits concentration. Before entering concentration, amid very noisy sounds, the mind can quickly settle down; the noisy sounds grow fainter and fainter, and then can no longer be heard, and the person enters concentration.

The Buddha taught the Huayan Sutra (Avataṃsaka Sūtra) and the Kṣitigarbha Sutra in the desire realm heaven and the form realm heaven, and then they were transmitted to the human world. The other sutras were all taught in the human world, so that heavenly beings could descend to hear the Dharma, and ghosts, spirits, and animals could also hear the Dharma, thereby liberating more sentient beings.