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

[Jojoya]

Then again, it's hard to uncover secrets when they don't exist. All areas I've find in open world that didn't have a quest or FATE in them were empty. There is no discovery unless there's something telling you to go and do a thing, and then it's just crossing that thing off a list.

There aren't even any secret FATEs anywhere, since they all just randomly appear on the map.

Can't speak for other games but WoW really wasn't any better when it came to discovery because spawn locations for treasure chests and the like were data mined and botted (why Blizzard ended up removing a lot of them over the years - there was no point when the bots would get them). There were the treasures phased at the character level that could only be collected once - and most players just looked them up on WoWhead to save time. Even if a player collected them via random encounter, the need to explore stopped once they were collected.

And actually there are "secret" FATEs in this game. They have spawn conditions that must be met. They will never appear on the world map when active, only the mini-map when you are in close range. It's not a huge number (only 2-3 per expansion) but they do exist.