How do people reconcile the "worker shortage" with 20 year lows in unemployment?
Because "unemployment rate" is a flawed measure reported by the media to make recession economies look better than they actually are.
The U-6 real unemployment rate includes the underemployed, the marginally attached, and discouraged workers. It's usually much higher than the U-3 unemployment rate, which is the rate most often reported in the media. The Bureau of Labor Statistics only counts people without jobs who are included in the labor force for the U-3 rate. They must have looked for a job in the last four weeks to remain in the labor force.
https://www.thebalance.com/what-is-the-real-unemployment-rate-3306198
The BLS commonly cites the U-3 unemployment rate (defined as the total unemployed as a percentage of the civilian labor force) as the official unemployment rate. However, this definition does not include discouraged unemployed workers who are no longer looking for work.
Other categories of unemployment include discouraged workers and part-time or underemployed workers who want to work full time but, for economic reasons, are unable to do so