Agreed. But allow me to clarify:

Having job does not prevent you from looking for another. One that pays more, is closer...

In some (many) cases, it practically does. Looking for a job requires time and effort. That job you don't want, but unfortunately need, might make you exhausted, or allow no time off for job interviews, or demand employee loyalty and actively discourage you from leaving.

You might stumble on a good opportunity despite all of the hurdles above, but that's far from certain. Instead, you would have to quit the current job, before looking for the next one. And that might not be an attractive option.