America needs to have never-ending perpetual wars to sustain its own economy. If we woke up tomorrow and there was just world peace, and America got rid of its military budget millions of people would probably instantly lose their jobs.

That's the ultimate reason. They could just as easily declare war against Venus and spend hundreds of billions of dollars sending rocks into space and it would have the same net effect. Actually it would be a bit more positive because to my knowledge nobody's really living on Venus right now.