In some ways, the Founders were delightfully naive. Kings also already existed, and they'd just thrown off control of one.

Many Founders believed a standing army to be an inherently corrupting force (if you have a warrior class, sooner or later they will make you go to war) and thought America would be better without one. Whether or not that could have been true in the abstract became irrelevant because the need to perpetually defend territory meant that there was never an opportunity to disband the Continental Army.