What is the justification for declaring someone who seeks help from the dead regarding matters a living person is capable of as a disbeliever, given that the most this could be is likening the dead to the living, not likening them to God?

The deceased cannot do anything for themselves, let alone for others. Therefore, asking them or seeking their aid resembles those who ask other than Allah for what only Allah can provide. Seeking aid from the deceased is a single type, unlike seeking aid from the living, where a distinction is made between what only Allah can provide and what others can provide. Seeking needs from the dead, seeking their aid, and turning to them are the root of idolatry in the world. The work of the deceased has ceased, and they possess no power to benefit or harm themselves, let alone those who seek their aid, ask them to fulfill a need, or ask them to intercede with Allah for them. This stems from their ignorance of the intercessor and the one interceded with. The deceased needs someone to supplicate for them, show mercy to them, and seek forgiveness for them. This innovation, which is seeking aid from the dead, asking them for needs, and seeking their intercession, is but a remnant of idol worship, just as claiming equality between the living and the dead is something rejected by both religious law and reason. Allah Almighty says: "And not equal are the living and the dead. Indeed, Allah causes to hear whom He wills, but you cannot make hear those in the graves."