Because military allies are more likely to trade with each other.

For example, USA and South Korea have a Mutual Defense treaty, they have a free trade agreement, they also have a tax treaty. There might be a bunch of stuff I don't know about. If the US threw away its Mutual Defense treaty with South Korea like you're suggesting, maybe the other treaties and agreements with Korea don't change... or maybe they do, I'm honestly not sure.

The free market loves Just In Time logistics, because it saves money on storage, but the downside is a lack of resiliency, so whenever there has been supply chain shipping disruptions, US companies, especially the auto manufacturers, struggle without micro processors from Taiwan. This is exactly why the CHIPS act was passed, to encourage microchips to be made in the US to decrease the corporate dependency on Taiwan... but it'll take a few years to really come into play.

Intelligence-sharing really helps US companies. Corporate cybersecurity teams interact with US intelligence agencies to both alert them and be alerted on cybersecurity threats and fixes. Some US companies are attacked 100s of millions of times every second. I don't know for sure, but I imagine the US intelligence agencies are also interacting with the intelligence agencies of its military allies to help all allied companies. I'm sure you know that the North Korean state and Russian private hacker groups attack outside companies relentlessly.

I think America has something defense treaties with something like 50 other countries. If it declared all those treaties null and void then I'd say that's likely to have unintended consequences regarding trade, etc.

Is the US the world's richest country that chooses to spend a huge budget on its military, or is the US so wealthy because it has the world's strongest military? No idea myself, but I'm sure valid arguments could be made for both sides.