With all Australia's hard border closures and mandatory hotel quarantine, we've still had the UK Covid strain get out into the community. A cleaner at a quarantine hotel caught it from a returned citizen in quarantine (they're still not sure how), and spent 5 days in the community while infected. So far, it looks like we got very lucky, and the only person they've found that she infected is her partner.
From my point of view, the US (and UK and India and many other countries) have a categorically different problem to Australia (and New Zealand and Singapore and Taiwan and a few other countries).
We've _largely_ got things under control in Australia. Most of our cases in the last ~3 months have been hotel quarantine, with worrying incidences of outbreaks where it's escaped from quarantine but been tracked back and identified. Sydney currently has 3 clusters, with about 200 locally acquired cases between them in the last 4 weeks. I'm pretty sure every single one of them has been contact traced back to it's source of infection - and that contact tracing has also proactively got forward to get a very significant percentage of close contacts of infected people contacted and tested, and the places they visited while infectious identified, contact traced where possible (lots of venues are required to have covid checkin so people can be contacted an informed if they've potentially been exposed) - and any venues where they are not confident the covid checkins are comprehensive they publicise fairly widely and strongly encourage testing for anyone who visited during similar times.
This _only_ works because the numbers are so low. It all went to shit in Melbourne when a quarantine hotel breach happened and it got up over 700 cases a day, the contact tracing ability totally fell apart, and Melbourne went into a hard lockdown for several months to get it back under control.
In the US? Our approach just won't work. The virus is out and prevalent in your community. You have no hope of comprehensively contact tracing all of your positive cases, or identifying and mitigating all the venues they were in while infectious and contacting all the people who are at risk of exposure. The scale of your problem is totally insurmountable using the tools that work for us. If vaccination and/or herd immunity doesn't work out soon? You people might be totally fucked. ~1% of you will probably die from the virus, and quite likely a lot more will die from your medical system breaking under the load.
Sadly, I suspect it's like gun control. (I know, I know. Touchy subject, but hear me out - and please accept that I'm not trying to attack gun owners here.)
In Australia, there are few enough guns that reasonably strict licensing for owning/keeping/selling guns _mostly_ works. We managed to restrict "assault style" weapons early enough to have been able to do it before the prevalence of semi automatic weapons made it impossible. Gun violence here is several orders of magnitude lower that in the US by any reasonable measure. Not zero, but rare enough that "gunshots heard" is still newsworthy here.
If the US decided to try and get to that - it'd be a multi generation process to change attitudes and perceptions, and then to remove all the weapons currently in circulation. I don't believe there's the political or social will to even consider starting that. Your solutions to gun violence are fundamentally going to be different to Australia's.
(From my perspective, the news that my nieces regularly do "active shooter lockdown training exercises" in schools was inexplicable and horrifying. I kinda understand your historical "right to bear arms" - but I can't bend my mind into understanding how that's ended up at "there's nothing we can do about disgruntled teenagers getting hold of AK47s so we'll just teach our children how to hide under school desks and not make any noise, hoping that 'playing dead' will help them avoid getting shot". Not that I'm claiming too much moral high ground here, the most recent mass shooting in New Zealand was by an Australian. We have our fair share of fuckwits here too. Our main advantage is that most of those fuckwits do not have access to firearms when they fly into a murderous rage.)
Because the largest driver of infections isn't foreign travelers. It's far and away due to the people who are already here.