One of the major contributors to US revenues had been corporate taxes. With globalization, more corporations are storing revenues off-shore, at least off-shore from the perspective of the US. The total amount held off-shore is approximately 10% of US GDP.
US multinationals in the Fortune 500 doubled the assets they hold in foreign subsidiaries between 2008 and 2014, taking advantage of loopholes in US and foreign laws to keep $2.1 trillion out of sight of tax authorities, a new analysis shows. – Quartz
The largest revenue reservoir is Apple with $181B. One of the aspects of globalization has been the ease with which currencies and assets can be transferred, and that with almost 200 countries there will almost always be some country offering tax haven status.
(Click on the chart for the link.)