Unemployment rate is at record lows. Employment rate is back to where it was pre-pandemic. It isn't that a huge part of the labor force didn't come back. It's that they got better jobs by shopping around after the pandemic.
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The employment rate shows percentage of the labor force that is working. It does not reflect that the labor force itself has shrunk. People have left the workforce for a variety of reasons, including retirements (the boomers are aging out). Some people are simply deciding not to work.