The US was the first large scale modern democracy. Of course it has design flaws.

Parlamentarism, as we know it now, had only been recently established in the UK in the 17rh century.

Contemporary to US early democracy were absolutist monarchies based on aristocracy. Separation of powers envisioned by Montesquieu, Rousseau‘s social contract, were still new political ideas. The federalist papers and later US constitution were cutting edge political theory at the time.

It’s very impressive that the US has lasted so long actually and was able to adapt. The French established their first democratic republic later and were unable to create a stable state.