My guess as to why the US is hesitant to drop the test requirement is that although the horse has left the barn with respect to the known variants, if the US drops the test requirement and a new variant with surprising characteristics pops up a few months or weeks later, the US would face a public outcry even louder than what we have now if the US were to try to reinstate a test requirement. In other words, once we drop it, it will be that much more difficult to reinstate it. Just my guess. That said, I think it will not be long before the US bows to public/airline pressure (or a court ruling) to drop it. And if a new variant catches us off guard, we'll just have to deal with it in some other way. Maybe we'll have a better antiviral med by then. I'm getting off topic here.