My experience was different.

1. It's a way to (pointlessly) delay adulthood.

adulthood != working for a company

Developing the next CRISPR, detecting gravitational waves, or building a stellator is a pretty grown up thing. In the end there is a non zero chance of achieving or contributing to something exceptional during your 20s (depending on your field).

2. Fear of the job market.

Well said. No other entity is recruiting young undergrads like academia does. Only topped by consulting firms, maybe.

3. Don't know what else to do.

The fellow students I encountered (~30) were mostly driven/obsessed by curiosity (2-3 exceptions maybe).

4. Magical thinking

Academia problem or society in general? This pattern prevails everywhere.

disclaimer: physics PhD