Except for 99.9% of those millennia, humanity had very different challenges than it does today. One of the biggest being effective communication between people. There's a reason the U.S. constitution talks about sending delegates to Washington D.C. Another being an abundance of basic necessities that simply did not exist when Voltaire was writing. Increased globalization, vast changes in technology, bitcoin, AI, automation, social media, etc. Why would you assume political theory from even 100 years ago holds up in the face of all those differences?
And political theory, like history, tends to be written by the victors. What percent of all laws ever written were about maintaining the status quo for the people currently in power? It's a lot; I wouldn't be surprised if it's over 90%.
There's no reason to believe that humanity has "figured government out" by now. There's still plenty of room for experimentation in what works best in today's world. For example, why can individuals not collectively draft legislation, in the same way they can draft Wikipedia articles or open-source software? What would it take to make that happen?