system
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human
[Lord of the Rings] Both Gondor and Rohan have a period where their respective rulers at the time of the books were "incompetent" - why weren't there other councillors or nobility to help rule the kingdoms?

At the time of the books, both Theoden and Denethor II have long periods (*edit* these periods may not have been VERY long after all, but I think the question is still relevant) where they their rule was plagued by incompetence, possession, and paranoia to the extent that they were working against their own kingdoms. They were like modern "Manchurian Candidates". Theoden was largely advised by Grima, a non-family member. How did this power dynamic emerge, and why was there no check or balance? It seems strange that Grima, and not other Marshals or nobles, would help rule the realm - especially as both the Westfold and Eastfold were losing to "the enemy". Theodred, Eomer, and Eowyn comment in the books that Theoden was visibly not well or healthy... yet only Grima helps to rule the land?

I extend the question to Gondor as well, given that Denethor seemed to make similar questionable decisions with literally no checks or balances from other nobles or councillors of Gondor, despite the Stewards deriving their power from being the Chief High Councillors.

It seems as though both Rohan (Kings) and Gondor (Stewards) had a hereditary autocratic/absolute government. I find the quick evolution into absolutism a bit extreme given that both kingdoms were ruled by dynasties with no ties to the more legitimate Numenorean dynasties, who would have claims to Arnor and Gondor. But, given the long-term, hereditary dynasties of both Rohan and Gondor, how could other blood relations or nobles not help rule the land to prevent their imminent collapse from multiple threats?
gpt
Well you make a small error in thinking that Denethor was an incompetent ruler. In fact he is was a very capable ruler and was able to hold back the forces of Mordor without any help from any of the other realms. Other nobility didn't step in because up until the very end, he showed he was a wise, capable, strong, if proud leader. But he was being slowly misled. He lost hope. He lost his first born son, his second son was on deaths door, the armies of Mordor were overrunning his own people, and Sauron showed him a vision of Frodo being captured (it was a distorted truth, but Denethor knew Frodo had the ring and now he thought that Sauron had it and all was lost.)