>If it was so clearly ineffective, why does it get challenged more often and replaced?

I supposed you meant "why doesn't it get challenged"?

Well, look at how long it took for a democratic/Republican system to appear and survive. The French 1st Republic was immediately at war with all of Europe (I am not talking of Napoleon at all here, it was before that, when the French King was executed).

Nowadays, good luck getting any kind of financing with an "alternative" governance model. The banks and investors will either refuse or edge by pushing higher return rates on you. The whole system is conservative.

The adage "democracy is the worst system, apart from all others" only becomes true long-term. There are plenty of short-lived democracies back to antiquity, in the middle of the middle ages, during the Renaissance, the XIXth century... All stamped down by "more efficient" dictatorial empires... That aren't here anymore. You can expect the same in the even more cutthroat corporate environment, where fitting the system buys you leverage.

And don't get me stated on startups: most of them seek only an exist strategy. Very few challenge any existing behemoth. They are basically externalized R&D.