A civilization is created by its best people: the best scientists, engineers, generals, statesmen, etc.
This happens because a meritocratic society of some sort is required for a civilization to arise.

In a democracy, the control over the fate of a civilization is handed over to the average voter.
The problem with this is that the average voter was never capable of creating his civilization, much less understanding how it functions.

Civilizations are necessarily the creation of their very brightest members: they represent a stretch of the human potential to a new plateau.

They can only maintain their peak achievement as long as the brightest keep making decisions in the specific areas they have demonstrated mastery.
When the builders of a civilization hand over the reigns to the average member, it is like a pilot handing over controls to their passengers.
The passengers never had the ability to take their plane into the air, and will inevitably steer it into the ground.
As a collective, voters cannot produce anything, their sole power and common denominator is the redistribution of the product of civilization from one group to another.

This is why all democracies tend to devolve into bankrupt welfare states.