4. For loving-kindness should not be developed at first towards the following four kinds of persons: an antipathetic person, a very dearly loved friend, a neutral person, and a hostile person. Also it should not be developed specifically (see §49) towards the opposite sex, or towards a dead person.
7. But if he develops it towards a dead person, he reaches neither absorption nor access. A young bhikkhu, it seems, had started developing loving-kindness inspired by his teacher. His loving-kindness made no headway at all. He went to a senior elder and told him, “Venerable sir, I am quite familiar with attaining jhána through loving-kindness, and yet I cannot attain it. What is the matter?” The elder said, “Seek the sign, friend, [the object of your meditation].” He did so. Finding that his teacher had died, he proceeded with developing loving-kindness inspired by another and attained absorption. That is why it should not be developed towards one who is dead.
Path of Purification, Ñāṇamoli translation, pages 291-292.
Is there any explanation for why, if the object metṭā is directed at does not exist, then access concentration or beyond does is not achieved? It's sounds like, to reach access, you somehow need to 'connect' to the object(person), and therefore if the object doesn't exist, you don't get there? After all there are other mediation subjects that are not living beings (e.g. kasina) and they can lead to jhāna. So as long as one can generate metṭā in themselves, why does it matter if the person it is directed at really exists or not?