Here's a valid reason to quit:

Republicans who are anti-vaccine and anti-mask have been screaming and physically assaulting employees who tell them they must wear a mask.

Last week, a store security man was violently stabbed in his forehead and body. For telling a man he must wear a mask. I'd quit, too.

That we don't have an answer suggests that the real reason may be disturbing. MSM says it's because people took stock while they were in isolation. That seems doubtful. Maybe the wingnuts are quitting because the actually realize they're at risk working as they have. Delta is the only thing that really changed, but I'm sure I'm giving them too much credit.

They're tired of bullshit. Plain enough?

No one wants to work 24/7 for shit wages anymore..

I give up.

They had a chance to look around, catch their breath, and realize they were running themselves ragged for Peanuts, after gas/car, wardrobe/fast food/taxes/child care, etc etc etc.
Why do that to your & your loved ones? Wear old clothes, make home-cooked meals (that you had time to find the sale prices for the ingredients) and grow kids with better values than "the latest" sneakers/phones!

They out of necessity realized they don't have to work anymore. Lots of people have saved every dime, invested well, their paid for house is worth 3x what they bought it for. Mostly baby boomers during the lockdown realized they'd like to make the situation permanent.