2 parties is the inevitable outcome of FPTP, there’s no legalistic enforcement of 2 parties, it’s simply the most viable strategy in this voting system. They very quickly became the biggest and out-competed all the others* (see how the Whigs fell, the conservative Democrats split in north and south and the progressive Republicans rose to prominence) Washington was right to deride the 2 party system but the framers were building an electoral system long before we had some extremely serious mathematics done about making voting systems more fair, and also proving that no voting system can be perfectly fair and satisfy all fairness criteria.

And they also weren’t one homogeneous group either, they all wanted different things and came to compromise about how to go about doing it, some wanted a centralized army to stand up to outside forces, others saw that as a risk, capable of waging war internally. They were working off of the collective knowledge of the Greeks, Romans, and more contemporary writers like Voltaire, Hobbes, and Rousseau. They didn’t have the next 250 years of political philosophy that would develop that we know today.