When you hire a new grad, and then spend a year getting them all up to speed with the process and CI system and code reviews and familiarization with the code base and they're just now starting to take on the responsibility for some of the code... and they leave a year and a day after you hired them.
And so then you hire another new grad... and {repeat}... and they leave a year and a day after you hired them.
At this point, management is a bit frustrated. They give it one more go... and they leave a year and a day after you hired them.
And now, when that position is opened up again, it is no longer an entry level position but rather a mid level. We forgo the extensive onboarding and gradual ramp up for a new grad and instead have a mid who should be able to understand the nature of a CI system, issue tracking, and so on and be able to start being a positive contributor in a month or two and so if they also leave a year and a day later, we've had several months of them being a positive contribution on the team.
... and now its just that much harder for a new grad to find a job.
With new grads, we can't hold 'em at all. For people who came in with 5+ years of experience, the average tenure here is 15 years. (Seriously, I've got 200 hours of "manager makes sure you use it" vacation, sabbatical accrual, an honest to goodness funded pension, and one of the better health insurance policies).
That happened to a report of mine once, obviously with a previous mutual agreement, but some recruiters mentioned that trend to me as well.
I think having a set date to leave is healthy, especially at this point in someone's career. You just do your job and you're rewarded with money and experience. You shield yourself from toxicity in a way, since don't want to depend on the job for having "best friends". Also you don't play office politics since you know it's ephemeral.