I like the other employment 3 year problem.

In this the company fills jobs with keen but cheap young people to save money. These people work hard, get experience and take qualifications. Three years later you have a cohort of young people with excellent CVs and qualifications above their peers... that the company can't afford to pay the market rate for! Then it loses a whole cohort of great staff.

I suspect many companies factor in that young people are the most likely to churn, so don't expend much effort retaining them