At a certain point they're beating a dead horse. Outside of graphical updates (which I thought the cartoon-y look of the leaders in civ 6 was a huge downgrade), the core gameplay is still mostly the same throughout the series.

I watched a video on civ 7 and it seems like they really tried to shake up a lot in the game, I think for this reason that they needed to try something fresh to stay relevant. But really this is to its detriment rather than benefit.

I'm not sure if the three age thing is to "even the playfield" on those marathon long sessions when one civ runs away with the ball so to speak, but really that's one of my favorite parts of the series. Like it's awesome to take out some cavemen with navy seals or launch nukes when everyone is cowering in fear. If everything gets massively reset, then why even try to get ahead? I've not played the game so there could be more nuance but that's my general impression.