Why should they? They're typically underpaid and in not so great conditions, but even if they aren't, why should they feel shape for quitting for a better job?

Why should they? They're typically underpaid and in not so great conditions, but even if they aren't, why should they feel shape for quitting for a better job? If you fulfill your contractual obligations, you don't owe your employer anything and leaving any job for a better one is totally not something to be ashamed about. If anything, shame on the employer for doing a bad job in retaining their employees.