Dark Events are not the same (i.e. don’t occur at the same time, tied to the same Guerilla missions) each campaign and can change things radically. There are also skill tree randomizations, research boost randomizations, mission randomizations. You will never step in the same campaign twice (Heraclitus).
How much these things change your own “grand strategy” is debatable, I suppose, but XCOM 2 (especially WotC) does not suffer as badly from this “sameness” as you seem to imply, imo.
There are also choices that you can make, mods to campaigns (including difficulty settings), soldiers, rule systems, that affect things dramatically.
When vanilla came out, I thought it had less replayability than Enemy Within for sure. Now, with War of the Chosen, I’m not as convinced.