For comparison, Call of Duty: Warzone regularly gets 30 million players.
1. Free to play. 2. Marketing budget in the 9 figure range. 3. Comes from a long established series with ~20 games. 4. Available on all platforms.
I'd be willing to bet there's more of an appetite for complex games today than when Civilisation was released from an absolute standpoint, just that there's also more people playing games in general so from a relative standpoint there's less.
COD sold 75-80 million copies [1]. Getting 30 million regularly implies that one of third of them are actively playing at the same time. That seems rather unlikely
[1] https://www.gamespot.com/articles/call-of-duty-warzone-sees-...