Why do sbag employer/interviewers penalise you for job hopping crap jobs?

Employers are looking for reliability in a potential employee, and job hopping demonstrates that you will jump ship as soon as you find something better. Whether you want to work or not is totally irrelevant, because they want to know that you'll stick with a job even when things don't necessarily go your way, for the good of the whole. Not a nice thing, but the truth just the same.