According to the US government, there are several reasons why it is the global leader in defense spending by a large margin. A good portion of the budget is spent on our military because the US is sort of the world’s police force. After World War 2, we somewhat took responsibility for stability in Europe, Asia, the Middle East, and all parts of the world where conflict might occur (so pretty much everywhere). Other countries like China and Russia that also spend a lot on their military have chosen to keep their military local, unlike how the US deploys troops across the globe to attempt to keep peace.
The reports that the public hears about may also be misleading when trying to figure out exactly how much money is spent on the military. The US’s defense budget in 2019 is reported $686 billion, but that includes the costs of paying corporations for contracting jobs around government buildings such as the Pentagon. In 2016, over $300 billion was spent on contracts between the government and other companies, which means a good portion of their budget was not directly spent on the military.
While some of the budget is not given to the military, the majority of it still is. The reason that our army costs so much to support is that the troops are volunteers, which means that they cost more to keep around than troops in other countries such as Russia where citizens are required to serve some time in the military.