I'd like to add another from the perspective of the hiring manager, now that I've had a few years experience in that role.
It's been my experience that people generally produce their best work after the first year. It takes a while to get to know any business: what its strengths and weaknesses are, and it's only with that understanding that someone can identify areas to make meaningful contributions.
If someone's tenure is consistently around their one year mark, it's unlikely that they've really ever expressed their full potential in a professional setting. They leave before things get really good.
Hiring is an investment - less what the candidate can do today, but what they can contribute over a more substantial period, given the right encouragement.