When the constitution was drafted, time delays necessitated that the military and the President act autonomously for the most part. Congress couldn't convene rapidly enough to expect them to have much say in all but the largest and most drawn out conflicts.

The War Powers Act is codifying this implicit power. The framers never considered a situation where the President and Congress is watching a conflict play out across the world in real-time and making decisions. So it's literally impossible to say how they wanted this to be handled. That's why they gave Congress the powers to write laws.