Why does everyone run ancient Postgres versions?

Because the actual process of upgrading Postgres is terrible.

I say this as someone who absolutely loves using it, but the actual process of upgrading Postgres is something that takes significant downtime, is error-prone and you're often better off just dumping all the databases and reimporting them in a new folder. (A good idea in general since it'll vacuum and compact indexes as well if you do it, combining a few maintenance routines in your upgrade path.)

It requires having the previous version of Postgres installed, something which can mess with a number of distro policies (not to mention docker, which is the most popular way to deploy software that will typically rely on Postgres), and unlike most software with that issue, Postgres is software you want to be managed by your distro.

Therefore, most people only upgrade by necessity - when their distro forces the upgrade or the version they're using reaches EOL.

This! I already mentioned this a while back and was basically berated (by someone that seemed like a dev/close to dev) that current setup is just fine and because of postgress complexity (extensions) it has to be done that way... and while I like postgress a lot it's quite annoying that the upgrade is such a terrible experience... :|

I suspect it’s also a bit of stockhold syndrome on behalf of admins haha

Or better yet - "guarding the job", i.e. making their job very important ;)