so why the devs never use 90% of the content for end game?

[Y2K21]

I don't know ... exploration in the MMOs you've mentioned is pretty, um, down on the list of things to do. And if it has some rewards to make it worth doing, there are add-ons (WoW/ESO) to tell you exactly where to go, or you can look it up on wowhead or the ESO/Lost Ark equivalents, so it's not really "exploration". It's making a list and checking it twice.

I'm assuming Theodric finished the sightseeing log without consulting one of the many websites that list every one of them.

This is like saying SE should stop making Savage Raids because you can find guide videos on Youtube. Or stop making Sightseeing logs, hunts, etc. because people will more than likely use third-party tools to aid them in doing them.