It's a persistent myth because the institution is set up to perpetuate it. Everything bad is the nobles, the lords, the boyars, the merchants. But if the king, all-powerful and distant, only KNEW about these abuses...

If I can make decisions unilaterally, I'll be more efficient not having to seek as much agreement from stakeholders, as long as we assume I'll make good decisions.

I think benevolent dictatorship can exist but only for a couple generations at best, and that is also probably exceedingly rare.

Greed being a virtue these days and corruption running rampant probably lowers these odds.

And all rulers grades are still subject to whatever constraints and opportunities their situation places them in. Without Philip investing in army and drill, Alexander could never have done what he did. Also I'm sure having an external enemy to loot and enrich your people's is a big lever too.

I think the more interesting modern question is about democracy versus single party rule like CCP. If the big benefit of democracy is we get more and better ideas and efficiency through private industry, how does the Internet making all information globally free and the global economy change that? I fear democracy loses a lot of inherent advantage in the same way Chinese companies steal IP or copy other products.

They also have the efficiency similar to the dictators. They can much better execute 40 year plans without having to switch parties and priorities every decade. How does democracy beat that in the information age?