You have a valid point but the GB gets its authority using Zechariah 8:23, therefore their decisions have divine support.
14 The flocking of many humans to worship Jehovah with his people in this time of the end was foretold by two ancient prophets. Isaiah prophesied: “Many peoples will go and say: ‘Come, let us go up to the mountain of Jehovah, to the house of the God of Jacob. He will instruct us about his ways, and we will walk in his paths.’ For law will go out of Zion, and the word of Jehovah out of Jerusalem.” (Isa. 2:2, 3) Likewise, the prophet Zechariah foretold that “many peoples and mighty nations will come to seek Jehovah of armies in Jerusalem and to beg for the favor of Jehovah.” He depicted them as “ten men out of all the languages of the nations” who, symbolically speaking, would take hold of the robe of spiritual Israel, saying: “We want to go with you, for we have heard that God is with you people.”—Zech. 8:20-23.
15 The “other sheep” “go with” the spiritual Israelites in the work of preaching the good news of the Kingdom. (Mark 13:10) They become a part of God’s people, “one flock” with the anointed, under “the fine shepherd,” Christ Jesus.—Read John 10:14-16.