Ignoring persona's there are 220 different Leader/Civ combos you could theoretically meet in Antiquity but because of the way it's set up I bet e.g. Hatshepsut of Egypt shows up more often than by random chance.
And I think this the latter is partly important: the way the randomization works is that it picks one of the leaders, and then assigns a historic/geographic civ choice, except if all were already taken. So, Hatshepsut will always get Egypt unless someone else took it first, e.g., Tubman. Then Hatshepsut will revert to one of her alternatives, which is probably Carthage or Aksum. Hypothetically, if these are already taken by the leaders that "chose" first, she goes random. But you only see this for the European leaders currently, e.g., in a game with Machiavelli, Charlemagne, Napoleon, and Lafayette. And for the combinations, I'm not sure how important it even is for the feeling of repetitiveness whether you meet Augustus or Rome, Augustus of Egypt, or Augustus of the Han.
But that would still mean that you have 22 leaders to meet and play against. More than in civ V and VI at release - and with less in each game than in these versions likely as well (as you cannot have more than 8). The question remains why it feels more repetitive than in these games? I think it's important that no one is able to compare it directly, because you now come from V and VI with much more leaders after many years to VII with much less. So, it might be much worse if anyone started civ VI now with only the base game and having 10 civs+leaders in each game. But still, as
@Verified_Confection_Being states, the problem is there and recognized. A good explanation isn't there yet. And it
cannot be civ switching, because this has nothing to do with the fact that 22/26 leaders feel repetitive, which is a problem even in Antiquity.