> I've often wondered why import tariffs and/or local manufacturing subsidies aren't used to address this.

Partially due to WTO restrictions and feigned ignorance. You're right that it's non-trivial to properly assess the P&L of outsourced labor especially when most of the externalities (ex. emissions spreading across East Asia, population health) are not immediately identifiable. Globalization ignores these externalities in favor of short term profit. To put a price tag on such effects would be to acknowledge them, which nobody will do.