Why does everyone run ancient Postgres versions?

My upgrade policy for everything:

Significant security vulnerability? Upgrade

Feature you need? Upgrade

All other reasons: Don't upgrade.

Upgrading takes effort and it is risky. The benefits must be worth the risks.

Upgrading when multiple versions behind is significantly more risky than doing it when the update is relatively fresh.

Additionally, actions done frequently are less risky than actions done rarely, since you develop skills in performing that action as an organization - see high deployment frequency as a strategy of managing deployment risk.

This adds up to continuous upgrading being the least risky option in aggregate.