In other words, if globally synchronized growth was a real thing, and its effects on the US economy were likewise more than rhetorical, then why hasn’t US income accelerated?

Why haven’t Americans shaken off the effects of 2015-16 and started buying more cars again (especially cars, though sales of light trucks are no longer growing, either)?

To be stuck in this position with, outside of weather events, a downside bias in sales and production for such a long stretch (closing in on three years) illustrates the opposite concepts from a boom.