[ilammy]

Not just more than one employer, but some healthy number of available options.

You thought A is great but you think you can find better so you go to B. They turned out worse, you then tried C and those weren't great either. But now A won't hire you back because they already filled the position, and there are no other available companies doing the thing you want to do. Now you're stuck with inferior employment options compared to the choice of not changing jobs in the first place, unless you're willing to do something else entirely or start your own business.

Depends on the field obviously, but there are reasons why people might want to stick with a not-so-bad job instead of looking for a perfect one.

[robertlagrant]

Of course - and employers have the same issue with employees. We don't have perfect information, but having conversations with alternate employers' employees helps. Just try and find stuff out. It's not going to be perfect, but it's also hard to imagine a better system that can work in practice.