Soul Society vs Seireitei
So that if you compare Seireitei with the rest of the other areas in the Soul Society, you can see that it is the only place that shows signs of luxury and urbanization in contrast to the rest of the regions of the Rukongai.
As mentioned in the manga, some areas don't even have shoes to wear. Seireitei is described as a place where the Shinigami live, but the most accurate definition is where pure souls live. And precisely the noble families, the five great families that preside over the rest. Bearing in mind that the main branch of the Shiba family has long lived outside the Seireitei.
Briefly, to summarize to you my general idea on this subject, I can say, the forefathers, after they decided to divide the original world into three worlds, chose to remain in the realm of the dead, that is, the Soul Society, so that they can control several things, the most important of which is the flow of human souls from the living world.
It may be that the forefathers somehow manipulated the humans in the original world. For example, possibly erasing human memory so that humanity could start over. But, at the same time, they kept the civilization they had in the original world to themselves, then built the foundation in the form of a small city that began to evolve with time until it took its current form, which we know as "Seireitei."
One Million Years vs a Few Thousand
The notion that Shinigami clean souls, wear specific clothes, and behave in certain ways was created by Yamamoto in the Gotei 13 a few thousand years back. You can see a discrepancy in the timeline of events here, right? In the Bleach series, how can the history of the Soul Society be condensed into a few thousand years?
The main issue with Kubo's storytelling was how he handled the timeline! Since we know the history of the Shinigami is a million years old but limited to a few thousand, I believe the time skips were a mistake. What are your thoughts on this? Do you have any ideas on how to fix these plot holes?