Why not is a lengthy answer which has nothing to do with the rubbish that the hawk type right wing idiots spout.
First and importantly: Why did the world have WW1 & WW2. Because Germany and Japan were empire building. Why? Because without either genuine free trade or a growing empire, then an industrialised country cannot become wealthier and expand.
When America joined WW1 it was to leverage in it's fiscal policies whereby it created an empire via fiscal hegemony and actively pressured to destroy land empires, especially those of France and the UK.
Then we have political doctrines. Fascism comes in many forms; National Socialism was one, but today's Neo Liberalism is another. The US & UK and others are today Neo Liberal societies. For it to work countries have to constantly expand their sphere of influence in essence. The USA uses it's armed forces to bully the world, maintain and create influence.
Why doesn't the USA stop spending more each year on arms than the next top 9 spending countries put together? There is no profit in helping people, then think of all the jobs that would be lost ..... It is just another case of the sick, twisted joke that is America.
America is collapsing and may well take the world with it when it goes.