And it was never designed to be. It was always meant to be a republic.

We first were a confederation. Were your idea of a true democracy was more or less in place. The revolutionary war was won in 1783. The constitution wasn't ratified till 1789, and the bill of rights written until 1793. Before that the US had almost no central government, and each state was independent from one another. Had their own currency, banking system, laws, and military.

States still have a lot of that same autonomy today, but there was no central government tying them together. If the US went to war and a state didn't want to go, they wouldn't. A little more complex than that, but generally that's what it amounted to.

Having this type of system created a bunch of problems and came to a head when Shay's Rebellion happened. I won't go into depth about it, but mainly confederated Massachusetts couldn't fight off the rebels attempting to take over the state. Since the US was a confederation there was no central government the state couldnt call on for help, and all the other states more or less said 'meh sucks for you'.

This incident lead to the Constitutional Convention that wrote the document we still uphold today, and bringing in more of a centralized Federal Republic, and not a decentralized confederated one.

My ranty point is, we tried the whole true democracy thing and it failed. So we went to a Federal Republic, still very much democratic, but moved away from a true democracy.