Most of the vectors of COVID spread in the US have little to do with failing to close itself off from the rest of the world. If you have the political will, you can implement quarantine for international travelers, which will catch the overwhelming majority of externally-sourced cases.

The US did not do so, because it was too disruptive to business, and because good, sober governance has been an explicit non-goal for the past few decades, on both state and federal level - but especially in the last few years.