1) Sense of (in)security. There is the thought of, what if my investments lose a lot of value. What if costs suddenly go up. So the number keeps shifting up. Just another x dollars and I'll retire.
2) As I get older, the thought of retiring is also starting to lose appeal. Life's priorities change and preferences change. I wanted to be able to not work and travel the world. Now that sounds some what exhausting and unanchored. Personal situation also comes into play. Single vs those in relationships. Will the partner welcome the FIRE lifestyle etc.
3) There is a bit of how society will view a 30 something with no job and no plans to get a job.