The problem is they have to give three month's notice when quitting a job. When other companies or headhunters learn that you gave notice, they do their best to hire you away from whatever new job you took.

Our HR Department now calls new-hires every couple of days to keep them engaged so they don't take a job elsewhere. It sucks when you hire someone, plan things three months in advance, and.... they don't show up to the job without ever telling you they took a job somewhere else.