None of these 4 reasons (TD;LR: you waited until the exit interview, you assume mentors want to manage, you assume the natural ascension of developers is management and not higher-level IC roles, you aren't supporting them and their career) are why they quit.
People quit because the modern, zero-loyalty, everyone-for-hire company does not provide a long term strategy for providing intrinsic motivation (autonomy, mastery, purpose); they don't want to pay for retention and therefore don't provide perks for retention.
Until this changes, it should never surprise a manager why churn is high and average tenure is less than 2 years. If you are a manager (especially middle management - meaning you don't have the scope to make massive, company-wide cultural changes), it's best to look at your directs as someone to help them advance for their next job. I know that sounds counterintuitive, but if you lead with transparency on the landscape, you build authenticity and trust with your directs, which will go much further for your relationship while you are working together.
People quit because the modern, zero-loyalty, everyone-for-hire company does not provide a long term strategy for providing intrinsic motivation (autonomy, mastery, purpose); they don't want to pay for retention and therefore don't provide perks for retention.
Until this changes, it should never surprise a manager why churn is high and average tenure is less than 2 years. If you are a manager (especially middle management - meaning you don't have the scope to make massive, company-wide cultural changes), it's best to look at your directs as someone to help them advance for their next job. I know that sounds counterintuitive, but if you lead with transparency on the landscape, you build authenticity and trust with your directs, which will go much further for your relationship while you are working together.