A benevolent dictatorship is obviously the most effective form of government, the problem is that any dictatorship can be used for good or evil and no matter how benevolent your queen or dictator or whatever you call her is, she's gonna die eventually and now you're rolling the dice. If you get someone less benevolent, system's a failure. If you get someone just as benevolent but less competent and they get deposed, system's a failure. And that's assuming that the system even survives succession rather than devolving into old-school feudalism or capitalist 'democracy' or ancap utopia or whatever in a power vacuum.

The only way to ensure a benevolent government remains long-term is by spreading the power out enough that the system can shrug off the deaths and retirement of individuals and keep going along some plan that exists beyond any one person

Okay, you say, so don't have a dictatorship of one benevolent queen but rather of a benevolent group acting in the best interests of the people. And this has the same pros and cons, but both blunted. It's good at what it does as long as the group remains truly benevolent, but any organization can decay and become corrupt and, in a sense, die just like our hypothetical good queen.

So if we just keep following the logic down, we end up at the idea that the group responsible for the wide-ranging decisions affecting everyone's lives should ideally be accountable to, more or less, everyone. If the ultimate goal is for the government to act in the best interests of the people, it follows that it is the people who should be the best judge of how well the government is working. If it stops doing its job properly, it is the people who notice. So it should be in the hands of the people to correct it.