Just a crappy economy.
So what if unemployment is 3.9%? The U30 is meaningless on its own, that's why politicians cite it so often. The employment rate in the US has remained flat at around 57 to 58%. That means that if the unemployment rate has fallen, fewer people are looking for work, not that more people are working.
I would posit that a major reason for this is that unemployment and welfare benefits at the moment are quite generous and amount to somewhere (depending on a person's circumstances) the equivalent of $20 and $30 an hour pay. Given that, why would anyone take a job for less pay than they can get sitting around doing nothing?
Don't post the truth, Cypress wants to believe that the shortage of truck drivers keeping shelves empty and the shortage of workers at restaurants means so many folks quit to work at Amazon that there are just no more folks to fill the jobs.