That's not actually true. To give a silly example: does anyone (seriously) claim that 'true fascism' has never been tried?

Or: liberal democracy (I'm sure you can find a synonym that ends in *-ism) has been tried. It's been doing ok-ish. Obviously it has warts and all. But more importantly: approximately no one ever seriously claims that 'real liberal democracy' hasn't been tried.

Similar for constitutional monarchy, or 'social market economy', or dictatorships, etc. People can mostly agree that the real deal has been tried.