Elaborate? I don't play it but want to learn more.
My thoughts on why may differ from Alberan Civfanatic's. But for me...
Amplitude's combat system (which is used in Humankind but also Endless Legend) involves an army of, typically, 4-8 units, which often consists of a mix of infantry, archers, cavalry, etc. Each type of unit has varying stats, and it does a good job of encouraging a balanced force composition.
But in each battle, you go into a separate battle mode. In Total War games, this is on a separate battle map, and is arguably the main part of the game. In Amplitude games, it's still a tactical movement-based system, but takes place on the strategic map. It seems cool at first. I find it tends to be wearying as the game goes on. Too much tactics being added to a strategy game. In Civ IV, it's simple, send your armies against the other armies, if they have a lot of cavalry, hopefully you brought a lot of pikemen, but it resolves quickly enough. In Humankind, quite a bit of the game can be taken up by that tactical battle-mode.
Civ-switching was my top complaint about Humankind, due to its immersion-breaking nature (it also happened much more frequently than it will in Civ7, and without notifying you when other civs changed, which could happen on any turn). But the battle system was the one that slowed the game down the most without really adding enough to it in return.