In some cases (as in the case of somebody my friend knows) they were in the restaurant industry. When the restaurant shut down due to the "lockdowns" the person got a job driving for Doordash (or Uber eats, can't remember) and ended up making more money doing deliveries than working in the kitchen. When the restaurant reopened she kept doing deliveries and didn't return to her former job.
News stories and service jobs all seem to talk about this like it's a surprise.
A year ago (give or take) a whole bunch of companies laid off huge numbers of employees.
Now, they want to hire them all back, ideally for them at the same pay as before.
But, as pointed out above, it's not like those people sat in suspended hibernation for the last year, doing nothing just waiting to go back to the same job they were dropped from. Sure, some may have been able to go on unemployment, with expanded benefits, just waiting to go back where they started. But, some huge number of them found other ways to earn a living.
Those companies looking to hire back the let go work force aren't primarily competing against someone doing nothing looking for a job. They're competing against the replacement job those people found. A replacement that may pay more. May have less risk. Is probably less variable due to changes in COVID spread, after all they got the job and have been doing it throughout the pandemic now. Even doing nothing, if the extra risk of the job or other things like child care or extra pandemic expenses make returning to a low paying job a negative value instead of actually making money.
Not to mention all the workers who don't exist at all anymore.
None of this is a surprise that low paying jobs that immediately dropped all their employees providing very little job security and generally not great working conditions would be harder to refill afterwards. They kind of count on a steady supply of new entry workers showing up as fast as existing workers move on to better things. This was a massive push to existing workers to move on faster than new ones are showing up.