I don't know why, but why do I get the impression that the Soul King is not as powerful as people are making him out to be. Now, hear me out, if the Soul King was really omnipotent, then why was Soul Society so worried about Aizen infiltrating the Royal Palace and assassinating the Soul King. If he was so powerful, why didn't they just let Aizen go there and be defeated by the Soul King and in the process, save lives.
Your point is quite valid.
There has been many stories, manga's, animes and movies and even real military type groups with similar situations. I will make this short, its about the RISK and usually loyalty.
The Leader or someone of great importance may be as powerful as a god, but the enemy will have to fight through the ranks before they are able to get to the final point/person. They don't want their leader to be taken out no matter how strong or powerful their leader is. It also keeps the guards or in this case the "Gotei 13" sharpened and battle ready more often in case. Instead of just letting some random "Joe" who thinks he is strong enough to kill a god but is weak as a fly accidentally kill him by mere chance.
i.e. The Warrior Achilles. "God in the battlefield" as they say, was untouched and never cut, until he took an arrow to his ankle, then died.
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Also you made me think of something new about the Soul King. What if the being we saw is just a vessel for the Soul Crystal? He could be extremely weak physically, willfully powerful, but given eternal youth for his the time he puts forth in being its voice. So just as immortal as an Elven folk, they live forever but can only die in battle (sorry i compared to another fantasy type humanoid, had to have a reference to go off). OR the being we saw could also be just a manifestation and be the one thing that is older than Soul society itself and created everything with its own will.
Like everyone says Aizen kept referring the Soul King to "IT". Both him and Urahara tried to make their own type of "Soul King" as the Hōgyoku with Urahara coming the closest by mere imperfection. When Aizen had it and merged he kept saying it was by its "will" that he was changing and transcending to a higher being.
