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

[DPZ2]

FFXIV struggles immensely with that by virtue of having very little reason to engage with the open world.

FATEs.
Daily hunts.
Monster trains.
Materials farming.
Role-play.
Parties at the beach.
Sightseeing logs.
Screenshots.

There are things to do in the open world. That they don't appeal to those who are jaded in a "been there, done that, so I don't need to do it anymore" kind of way does not equal "[players] have very little reason to engage with the open world". The reasons are there already. Participation is optional.