Why is it in Sikhi that a human needs a Guru or “middle man” to merge with God?

Respected Brother/Sister,
Waheguru ji ka Khalsa, Waheguru ji ki Fateh!

In response to your question the undersigned will like to say that, during his discussion with the Jogis, Guru Nanak Sahib was asked a question, as to who was his Guru. His reply was that his Guru was Shabad (ਸਬਦੁ ਗੁਰੂ ਸੁਰਤਿ ਧੁਨਿ ਚੇਲਾ). So, human body of none of the Gurus was Guru. It was Shabad Guru always.

As understood by the undersigned, for Sikhs, Akalpurakh is Hukam (Divine Law) Roop (Form) and Guru Granth Sahib i.e. Shabad Guru is Updesh (Adviser or Teacher) Roop (Form) and spiritually both are embodiment of each other. In other words, as per Gurbani both are the same, since the Gurbani teaches us:-

ਗੁਰੁ ਕਰਤਾ ਗੁਰੁ ਕਰਣੈ ਜੋਗੁ ॥ ਗੁਰੁ ਪਰਮੇਸਰੁ ਹੈ ਭੀ ਹੋਗੁ ॥ ਕਹੁ ਨਾਨਕ ਪ੍ਰਭਿ ਇਹੈ ਜਨਾਈ ॥ ਬਿਨੁ ਗੁਰ ਮੁਕਤਿ ਨ ਪਾਈਐ ਭਾਈ ॥੪॥੫॥੭॥ {ਪੰਨਾ 864}

ਅਰ੍ਥ: ਹੇ ਨਾਨਕ! ਆਖ-ਗੁਰੂ (ਆਤਮਕ ਅਵਸਥਾ ਵਿਚ ਕਰਤਾਰ ਨਾਲ ਇਕ-ਸੁਰ ਹੋਣ ਕਰਕੇ ਉਸ) ਕਰਤਾਰ ਦਾ ਰੂਪ ਹੈ ਜੋ ਸਭ ਕੁਝ ਕਰਨ ਦੇ ਸਮਰਥ ਹੈ। ਗੁਰੂ ਉਸ ਪਰਮੇਸਰ ਦਾ ਰੂਪ ਹੈ, ਜੋ (ਪਹਿਲਾਂ ਭੀ ਮੌਜੂਦ ਸੀ) ਹੁਣ ਭੀ ਮੌਜੂਦ ਹੈ ਅਤੇ ਸਦਾ ਕਾਇਮ ਰਹੇਗਾ। ਹੇ ਭਾਈ! ਗੁਰੂ (ਦੀ ਸਰਨ ਪੈਣ) ਤੋਂ ਬਿਨਾ (ਮਾਇਆ ਦੇ ਮੋਹ ਦੇ ਅੰਧਕਾਰ ਤੋਂ) ਖ਼ਲਾਸੀ ਨਹੀਂ ਹੋ ਸਕਦੀ।੪।੫।੭।
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ਗੁਰੁ ਪਰਮੇਸਰੁ ਏਕੋ ਜਾਣੁ ॥ ਜੋ ਤਿਸੁ ਭਾਵੈ ਸੋ ਪਰਵਾਣੁ ॥੧॥ ਰਹਾਉ ॥ (ਪੰਨਾ 864)

ਅਰਥ: ਹੇ ਭਾਈ! ਗੁਰੂ ਅਤੇ ਪਰਮਾਤਮਾ ਨੂੰ ਇੱਕ ਰੂਪ ਸਮਝੋ। ਜੋ ਕੁਝ ਪਰਮਾਤਮਾ ਨੂੰ ਚੰਗਾ ਲੱਗਦਾ ਹੈ, ਉਹੀ ਗੁਰੂ ਭੀ (ਸਿਰ-ਮੱਥੇ) ਕਬੂਲ ਕਰਦਾ ਹੈ।੧।ਰਹਾਉ।

There are a number of Shabads in Guru Granth Sahib, which confirm this. Therefore, the Guru is not a middle man but spiritually embodiment of Akalpurakh.

The only difference is that the laws laid down by Akalpurakh have to be followed by all without any ifs and buts whereas the teachings of Shabad Guru are followed by some and not followed by others.

Those who follow the teachings of Guru Granth Sahib, become Gurmukhs (ਗੁਰਮੁਖ), get rid of vices, become one with Akalpurakh and Shabad Guru, achieve the stage of spiritual equipoise (ਆਤਮਿਕ ਅਡੋਲਤਾ) and spiritual bliss (ਆਤਮਿਕ ਆਨੰਦ) and finally achieve emancipation/salvation in this life itself (ਇਸੇ ਜੀਵਨ ਵਿਚ ਮੁਕਤ ਹੋ ਜਾਂਦੇ ਹਨ).

On the other hand, those who instead of following teachings of Guru Granth Sahib, follow the guidance of their mind (ਮਨ), become Manmukhs (ਮਨਮੁਖ), die spiritual death every now and then, waste their valuable life and go away.

Hope it helps. If you have any further questions, please do ask. If you find any deficiencies, please point out the same, for improvement in future.

Regards,
Your Brother