Thats more a player PoV.
DM might have a different opinion. Killing the king in personal combat may not be that hard.
King might be backed by a dragon, gid, a church etc
That king might be backed by others far more powerful, but that's not personal power. Those connections can be broken far more easily then actual personal power. The dragon might be reasoned with (before or after), a church might be shown that the king does not represent the church's ideals (and there's a better choice), etc. Alliances might be (easily) broken. That does not mean that other kinds of power are inconsequential, they do matter. But a church would think twice to oppose a 20th level character or an ancient dragon when their own forces lack such powerful
individuals.
In our reality heads of state have been assassinated by common individuals, with no regard for their own safety. When that's done, it's done, in a D&D fantasy world, such individuals could be resurrected or brought back from the dead by other means. A commoner trying to kill a level 5 character is pretty much impossible unless you start equipping them with stupidly powerful magical items that would cost an equivalent of $2 million (Necklace of Fireballs for example) in our real world currency...
What I'm getting at is that regardless of the indirect power someone wield, personal power also means a lot. A common Bandit won't be overbearing against a guard unless they are bluffing or really stupid. But a Bandit Leader might do something that would be plain stupid for a common Bandit, a Bandit Lord even more so... A wise noble might be careful against a Bandit lord, but an unwise one might lean too much on their indirect power and ignore the direct personal threat.
In our world a common street thug might think twice when they have to face a famous MMA fighter in a fistfight, but they would be way more confident with a gun and some distance. No matter how skilled the MMA fighter, they can't teleport and get to the gun, they don't have hp, they die quite easily when they can't use their skills. The D&D fantasy world is very different and a LOT of people tend to forget that.
I also think that's a huge disadvantage of the current D&D 5e NPC system where there are no longer leveled NPCs. While a hereditary noble might be an inconsequential 1/8th CR NPC, a first generation noble would be a whole different kind of animal... There's generally a
very good reason on why they were made a noble of the realm.
I would say that information is WAY more important in a D&D fantasy world then in our reality, there's only a small range of power an individual has, with a very hard limit. In the D&D fantasy world, the person dressed like a simple physician might be a level 10 Monk... When a Bandit Lord dresses like a common Bandit... You better find out before hand who this person is, and how powerful they are, either by magic or by reputation (which might be wrong)...
Hence what we would consider Chaotic Stupid in our reality, might not be Chaotic Stupid in a D&D fantasy world. Theoretically there might be many, many Chaotic Evil people in our world that don't act on their impulses because they are smart and wise, the risks and consequences are higher then what they are willing to pay...