If a deeply awakened Master dies and then is reincarnated why do they have to do shugyo to open their dharma eye in this new body?

Dogen Zenji became ordained at age 8 fulfilling his mother´s request by doing so. He entered Hiezan and his question was: If humans are already enlightened, why do they need to train and experience enlightenment ? Yet there was no one around who could give him an answer.

At this time there was Eisai Zenji in Kenninji who had just returned from China. Dogen Zenji went to Kenninji to ask Eisai Zenji his question. Eisai Zenji gave the word of Nansen Osho as a koan to him. Saying that rather than asking others about it, only one can realize it for oneself. The koan was: how come do the Buddhas of the Three worlds not know, yet the animals do know. Dogen Zenji took this question with him to China. He entered Tendo Mountain where he did training and experienced awakening under Tendo Nyojo Zenji. Taking this experience, he deeply understood that the eyes are horizontal and the nose is vertical. Dogen Zenji left great teachings behind, and until his death he would teach anyone who was still deluded.

Do you understand Dogen Zenji´s koan? Satori is not an idea, Satori is not your possession – it belongs to all beings. Having experienced satori, it is not about entering heaven and being self satisfied. Sentient beings are numberless, I vow to liberate them. Desires are inexhaustible, I vow to put an end to them. The Dharmas are boundless, I vow to master them. The Buddha´s way is unsurpassable, I vow to become it.