Couldn’t the CDC have forbade that aircraft from landing in the US?
Unfortunately, a lot of US quarantine laws were misused by idiots (”We’re going to quarantine you uppity n*****s so your disease of wanting equal rights don’t spread” - among other misuses) and were weakened or struck down by the courts as a result. Those that remain are mostly weak and at the state level.
The President *can* close the borders, but as we saw in the last two years the courts will forcibly reopen them. And he can’t do it ‘tactically’, i.e., just a particular person or flight.
Unfortunately, what happened here is what happened to a lot of our laws in the 60s and 70s - a bunch of Pollyanna idiots got ahold of it and decided that certain laws were outdated, or the situations they were designed for would never happen again. (Obama - “We will never need tanks to fight tanks ever again” - that level of stupidity.) And therefore they gutted or removed the laws.