Because American President's don't have the power to declare War. War isn't a meaningless word from a legal or statecraft point of view. If Trump's military actions are short/fast enough - less than 60 days, from memory - they can be declared as simple military operations, and that's totally fine. Venezuela is a perfect example of this (though, that raid is likely in violation of international law and treaties). Any longer than that time frame, and the President needs a new designation for their actions or they must ask Congress to declare a state of War. Other American Presidents have worked around this to keep military actions moving, calling them everything from "policing operations" to "counter-terrorism raids" to get around needing a state of War. But, if Trump wants War powers - to, say, call up a draft or postpone elections - he needs a state of War.