It’s hard to get good answers to that question, but my best but very uneducated guess from reading is that more people have been retiring than entering the workforce for several years now. That accelerated during the pandemic. Add in the increase in the death rates the last couple of years for however you wish to attribute it, and the reductions in *legal* immigration, and you have yourself a labor shortage. Most people who worked in service industries have moved on to other jobs. Some people have left the workforce entirely, but I think that’s only a small part of the story, and the extended unemployment benefits have long since ended, so clearly that’s not the cause of the problem either, even though that was the narrative according to many last year.
TLDR: a labor shortage has been slowly occurring last few years, pandemic accelerated it and made it worse.