Patience was not something he lacked. The Great One had spent all of his life as a Celestial exhausting archives and ancient Ruins to find any solution to his bottleneck. No matter how hard he tried, there was nothing.
He would hit a ceiling and his life would run out. He had screamed at the Heavens but it ignored him. Lesser races were free to find opportunities or suddenly be blessed by Heaven and Earth to advance after being stuck for millions of years.
Only when his body died and his soul made it to the spirit world that he found a chance. He observed how rebirth actually worked and his massive efforts found it. There was a tiny chance.
He could not do it while as a Celestial but has a soul, he had the chance!
A Celestial could not possess others. It was one of the restrictions but no one had tried to make a body for them. The Qi they needed was anathema to most processes. For some reason, other Celestials had not bothered to even try which was another annoyance.
