Perhaps most important to understand, the virus doesn’t magically disappear when the herd immunity threshold is reached. That’s not when things stop — it’s only when they start to slow down.
Once enough immunity has been built in the population, each person will infect fewer than one other person, so a new epidemic cannot start afresh. But an epidemic that is already underway will continue to spread. If 100,000 people are infectious at the peak and they each infect 0.9 people, that’s still 90,000 new infections, and more after that. A runaway train doesn’t stop the instant the track begins to slope uphill, and a rapidly spreading virus doesn’t stop right when herd immunity is attained.
If the pandemic went uncontrolled in the United States, it could continue for months after herd immunity was reached, infecting many more millions in the process.