Today, the game may have changed. The united states is normally credited with having the most advanced, largest and most powerful military in the world. But seems unable to do anything effectively with it.
US has the largest and most expensive stuff not the best or most advanced. In fact a lot of US equipment are already behind the leading technologies such as the F35 planes that are already at least 2 steps behind the defense systems.
Hell yes they're underpaid--why do you think the US outsourced all of its manufacturing jobs to China and various other Asian countries? It's because they can do the job dirt cheap.
Underpaid compared to what? To their cost of living? Not at all, and that is the only thing that matters. So you can't call it "underpaid".
I don't know much about China and their cost of living but one mistake that people usually make when comparing wages is that they compare it without considering the cost of living. For example if a worker in another country is receiving half the payment compared to an American worker then the cost of living for that worker (utility bills, rent, groceries, etc.) costs half in that country compared to US too.
For example gas in US costs $3.5 per litre, $5.79 in UK, $5.57 in Germany,... while it is $1.2 in China ($0.05 in Iran)!
To answer your question about "why", it is because it is ridiculously expensive to live or work in US as everything ins overpriced while it is not in China.