GyAni is similar to God(Krisna)'s own self.
Hence serving (or Poojan) a GyAni is equivalent to offering the same to the lord. This is also basis of Bhakti.
Ok, but why not Krishna himself?
Arjuna was anyway surrendered to Krishna. But through Gita, Krishna has made the message eternal. i.e. not specific to Arjuna or their life times. Had Krishna insisted Bhakti for his then personal form then it becomes too limited in availability. One day Krishna has to depart from earth, but GyAnis will always remain in some or the other forms.
That's why Krishna did not disregard other GyAnis compared to his then human form. In fact, he always acted in prescribed duties without trying to bend the social norms & rules. Depending on situation, he was quite humble. There is an event in Rajasuyika Parva, where Krishna arrives for the yagna and asks Yudhishtira to command him to take up any duty suitable for him. He too washes the feet of Brahmins. He had nothing to achieve but still (or hence) he acted into his Karmas, which were actually Akarmas. Because he was aware that, should he misconduct, everyone will follow him, leading to the destruction of society.
 GyAni = enlightened or self realized with transcendental awareness
 BG 7.18 — All these devotees are undoubtedly magnanimous souls, but he who is situated in knowledge of Me, I consider to be just like My own self. Being engaged in My transcendental service, he is sure to attain Me, the highest and most perfect goal.
 Krishna, extolling his virtues, said.--'Thou, O tiger among kings, deservest imperial dignity. Let, therefore, the great sacrifice be performed by thee. And if thou performest that sacrifice an obtainest its fruit we all shall regard ourselves as crowned with success. I am always engaged in seeking good. Perform thou then the sacrifice thou desirest. Employ me also in some office for that purpose, for I should obey all thy commands.'
Also in BRC Mahabharata, there is an episode where Krishna washes feet of Brahmanas.
 Akarma = non-fruitive actions in accordance of Swa-Dharma
 BG 3.23, 3.24 — For if I ever failed to engage in carefully performing prescribed duties, O Pārtha, certainly all men would follow My path.
— If I did not perform prescribed duties, all these worlds would be put to ruination. I would be the cause of creating unwanted population, and I would thereby destroy the peace of all living beings.