Even if it is manufactured and sold outside the country, lots of work inside the US helped to produce it (designing it, programing it, testing it) so shouldn't some portion of the profits still come back?
The bigger issue is that their offshore money is not only from offshore business but it does get "exported" as well. They set up a bunch of shell corporations and then one of their shell companies pays another for whatever reason they make up in order to move money from one country to another. For instance, Google US could take all their profits and pay it to Google Ireland for [insert any reason] and then Google US's taxable income would be $0 so they'd pay no taxes in the US. This is how literally every multinational corporation avoids paying taxes or at least significantly reducing them.