Efficiency, it's not insignificant.

With gas cars people don't really notice since it's easy to refuel, but we've probably wasted 10s of billions of dollars in fuel over the decades and all the co2 emissions that adds up to over it.

Edit: Just an example of numbers... In 2022 the USA alone used 135.73 billion gallons of gasoline. Take even just 1% of that and it's 1.357 billion gallons of fuel lost to aero drag, but it's likely more than 1%. US average cost of gas is $3.224 a gallon. So that's $4.378 billion in wasted fuel in 2022 and a large amount of unnecessary emissions, assuming this was all from cars (but it probably wasn't) . That's like if the entire USA didn't drive for multiple days. And that's just the USA. Imagine if the whole world just didn't drive for 3-4 days.

If the US version is not the same as the EU version there’s probably a regulatory reason they’re not using the EU version. Or maybe an EU recall is coming. I don’t know the specifics here, but generally if a US vehicle is different from its EU version there’s a regulatory reason.

Car makers would prefer if North American and European regulators could agree to a single standard for everything because it would simplify their designs and lower costs, as well as open more markets to niche vehicles that aren’t worth the investment to modify, but it hasn’t happened so far. There’s a mix of egos in play as well as legitimately different needs for different regions, but the legitimate issues should be resolvable if there was political will.

EDIT: I suppose it’s also possible the Chattanooga plant just was following a bad assembly procedure or sourced a bad part that isn’t an issue from other factories.