The number of resignations is still at a record pace a year later. On top of that, employers are feeling the pain of not being able to be ridiculously picky about new hires. As someone who quit last year, I think I know why work to rule is becoming such a big deal.
The unimaginative manager thinks that employees are either good or bad, lazy or motivated, loyal or deceitful. People can be all of these things at different times. I was more engaged when I had interesting work. I would pull back on effort when I was treated poorly.
Now that something like 1/3 of the US workforce has quit a job in the last few years, my guess is that those louder quitters are more likely the ones who put in effort, increased their skills, tried to make a difference and got shafted by their employer. The “quiet” people who stay at bad companies would be more likely to be comfortable where they are, probably because they're treated better at their jobs or the job is easier. They're less likely to take a leap to another employer, but they're also not killing themselves at their current one.
Employers only looked at the revenue and the employee cost. Even in a workplace with different levels of employee effort, some people naturally take on more or harder jobs. Companies are terrible at determining metrics, so they think everything is fine. Now that the people who tried to make things better are gone, managers are left with the workers who keep the status quo and now they're bemoaning that because they relied on the free effort of people with different levels of motivation.