Why are all Buddhas unwilling to stay and teach the Dharma when they attain Buddhahood in worlds similar to the Saha World?

When you attain the third or fourth fruit and have eradicated the afflictions of greed, hatred, and delusion, your state of mind when looking at sentient beings will be completely different from what it is now. There will be many things you will no longer say, because saying them would be useless—just like a university professor looking at kindergarten children, feeling that they are truly beyond teaching. When you become a Buddha and observe the multitude of sentient beings, all steeped in the afflictions of greed, hatred, and delusion, so utterly foolish, you will likewise feel that expounding the profound Buddhadharma to such sentient beings would be simply unimaginable—it would be even worse than playing the lute to a cow. When one is at roughly the same level as sentient beings, one does not feel anything particular about them at all; but once one transcends that level and leaps up to look down from above, that state of mind is something you all cannot understand or even imagine, and so speaking of it is useless. The Buddhas, Bodhisattvas, and sages have no kindred spirits and are very lonely; sentient beings cannot understand them, much less fathom the state of mind of the sages.