Why is the playthrough for Assassin's Creed Odyssey so long?

I just finished killing Pylades and solved the puzzle to get the treasure, but I have no idea how much is left...
I'm currently on the "Two Kings and a Rider" quest,
my playtime is already over 90 hours, and my level is 65.
But since more than 1/3 of the Cultists are still remaining, it feels like there's still a long way to go. Is it originally this long? lol
It's not that it's not fun, but I feel like I'm skipping more and more...