Commerce is a way to circulate wealth and resources, with trade simply extending that beyond national borders. And there are a variety of circulatory paths involved, some of which benefit a few, some of which benefit many, some of which ultimately transfer wealth from one group to another. Which is not to say that growth isn't a factor.
Cutting taxes for those at the top encourages investment, for sure, but that investment isn't occuring in this country, but rather overseas where a variety of factors improve profit margins when selling consumer goods back into the US market. Instead of the classic closed loop theory of economics, we have an open loop at the national level, where increased profits are increasingly invested elsewhere and where balance of trade deficits slowly transfer even more wealth out of the nation. When those funds eventually return, they aren't used to buy comsumer goods, but rather as investments in money-making propositions, further enhancing the wealth extraction effect. Even as the economy grows, true wealth concentrates into the hands of fewer and fewer entities, many of them foreign.
It's not rocket science, at all, but something based on the whole capitalist concept of ownership. When a company's domestic workers do a great job, allowing sufficient capital accumulation to open a new division in China and that new division in China becomes highly profitable, the domestic workers don't get a bonus, don't share in the success they've helped to create (beyond their wage+benefits)- they get laid off... because they have no ownership. The only way that the public, the workers, can express any sort of ownership is in terms of restrictions on capital exports, and as taxes redistributed in the domestic economy. Which is, in a democracy, their right.
And, of course, factory closings have a larger effect on the local economy- overall business activity decreases, capital dries up. The rust belt is testimony to this.
Eventually, the situation is self correcting via the mechanism of currency devaluation, which is what we're starting to see. We've flooded the world markets with dollars at a rate that the rest of the world can't properly absorb. The supply is greater than the demand, so the value goes down... but that devaluation doesn't undo the wealth transfer that has already occurred, but rather strengthens and entrenches it... As the dollar falls, the value of foreign holdings goes up in relative terms... Who wins? Who loses? the answers are obvious, as Craig234 points out, above...