Why do we say that it is disbelief to ask the dead to fullfil someone's need but we do not say that it is disbelief to ask the dead to ask Allaah? In both cases, the caller is calling/supplicating the dead, and in both cases, the caller is asking the dead for something which he can not do, and in both cases, the caller thinks that the dead can do this by the will of Allaah. For example, if he asks the dead to fullfil his need, like by saying, 'Help me,' he thinks that the soul can come and help him by the will of Allaah. Why is this disbelief while asking the dead to ask Allaah is not?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger. 

The scholars differentiated between asking the dead person to fulfill one's need, cure him from illness, give him something to which he aspires, or ward off harm from him, and between asking the dead to supplicate Allah in his favor. The first act is considered Shirk (associating partners with Allah) because it is generated from the belief that the dead person can control the affairs of the universe. The second is a means that leads to Shirk because the person does not believe that the dead can control the affairs of the universe; rather, he thinks that the dead can hear him and supplicate Allah for him just as a living person can supplicate for him. Shaykh Ibn 'Uthaymeen said, “Asking the dead to ask Allah and asking him to fulfill a need are different. If he asks for fulfilling a need, this is major Shirk, and if he asks the dead to ask Allah on his behalf, this is Bid'ah (innovation in religion) and a misguidance...”

Nevertheless, some scholars held that when a living person asks the dead to supplicate Allah in his favor, it may be considered Shirk in some cases:

Shaykh Bakr Abu Zayd wrote:

Regarding the issue of a living person asking a dead person to ask Allah for him, there are two branches:

1- When a living person asks the deceased person while away from his grave to supplicate Allah in his favor; this is considered major Shirk according to the consensus of the Muslims, and it is similar to the Shirk committed by the Christians, who associate Maryam and her son (Mary and Jesus), may Allah exalt their mention, with Allah in worship by supplicating them, believing that they have knowledge of people's actions, according to Christian claims.

2- When a living person stands by the grave of the dead person and asks him to supplicate Allah for him, saying, 'O so-and-so supplicate Allah for me to fulfill such-and-such needs for me.’ Muslims unanimously agreed that it is a religious innovation to seek an intermediary between the person and his Lord, and this leads to Shirk, supplicating the dead instead of Allah, and drives the hearts away from their Lord. However, this is considered major Shirk when the person asks the deceased for intercession with Allah in the way the polytheists (in Makkah) did by worshiping the idols as related in the verse: {We only worship them that they may bring us nearer to Allah in position.} [Quran 39:3]” [Tas-heeh Ad-Du‘aa']

Shaykh ‘Abdur-Rahmaan Al-Barraak said:

“One of the kinds of major Shirk is Shirk in supplication, namely to supplicate the dead and absent people to fulfill one's needs, calling upon them, seeking refuge with them at times of distress, and asking them for victory and provisions, regardless of whether this invocation is done near the dead person's body or not as long as the person believes that this dead person is able to help him and fulfill his needs. The doer has thus committed Shirk in Lordship and Shirk in worship.

However, if the person asks the dead to supplicate Allah in his favor while near his grave, believing that he can hear him, then this is a religious innovation and a means that leads to Shirk, as Shaykhul-Islam Ibn Taymiyyah underlined in this regard.”

Shaykh ‘Abdul-Lateef ibn ‘Abdur-Rahmaan Aal Ash-Shaykh wrote:

“It should be said to this polytheist, ‘If you supplicated this dead person believing that he knows your situation best and he is more able to answer your supplication and fulfill your need or is more merciful towards you than your Lord, you should know that this is plain ignorance and disbelief. And if you know that Allah knows your situation better and that He is able to fulfill your need and is more Merciful towards you than anyone else, why then would you supplicate other than Him? If you responded saying, 'This dead person's supplication would be more likely accepted by Allah than mine,' then you should know that this falls into the second category, namely when a living person does not supplicate the dead person or asks him to fulfill his needs, but asks him to supplicate Allah in his favor, like the Companions used to do with the Prophet, sallallahu ‘alayhi wa sallam, and ask him to supplicate Allah for them. It is permissible to do this with the living, not with the dead, be they Prophets or righteous people or anyone else. None of the Companions or the Taabi‘oon (generation following that of the Companions) did so, and no hadeeth was reported to this effect. Rather, it is a religious innovation and such an act has no basis in the Islamic Shariah...’” [Kashf Shubuhaat Ibn Jirjees - Unmasking of the Misconceptions of Ibn Jarjees]

Allah knows best.