Why? Let's start with:

1. More students than advisors, so a built-in pyramid scheme unless academia expands exponentially forever.

2. Fully backloaded reward system, so a built-in lock-in until the end, and the evaluation/reward isn't under your control.

3. For some, spend years doing work but only a small piece is relevant to the reward.

4. For some, zero actual training for the eventual academic job, if they get it.

5. It's a job where you do all the work and take all the risk, but are paid like shit and treated like a favor is done to "train you".

In other words, the whole thing is set up to destroy the self worth of a normal person and take away their agency and independent identity. Without coping strategies, I would expect them to develop mental issues.