Historically, Civilization game was always pseudohistorical. More than history, the game relied on the building and warring for your own empire more than historical accuracy. The different civilizations were never meant to represent the actual events and leaders were more like icons (think of Stalin and Mao in CIV 1), later they received unique attributes and personalities. These were not chosen based on their historical significance.
Also, in the CIV incarnations, different strategies were chosen. If you compare Civ5 and Civ6, it is clear that one strives for realism (in then-current graphics), the other chose more cartoony look. It went with the popular trends. Similar is the choice of Civs and their Leaders. America did not exist until 1776, Sumeria did not exist after 2000BC. What we see in Civs are not nations, it is always a cultural group brought together by ruler or similarity.
Did Gorgo lead all the greek Polis? Did Alexander raze all the former cities and grounded new ones instead? Did Saladin lead "all" the Arabians?
Your argument contradicts itself. Greece the civilization in Civ VI is not 'brought together' by ruler, quite the opposite: Sparta and Athens represented the two Extremes of the Greek Polis in the time of Pericles and Lysander/Gorgo: Athens the most egalitarian-Democratic, Sparta the most autocratic - militaristic. Neither of them accurately represent the majority of the Greek political entities and Gorgo never even represented the political leadership of Sparta - like too many Civ Leaders, she is a completely artificial Figurehead. And Alexander only razed a few cities (Thebes in Greece, Persepolis in Persia the two most famous) but he founded dozens, many of which are still in existence. Alexandria in Egypt is only the most famous among a long list stretching from the Balkans to what is now Afghanistan.
I mean, Near and Middle East inhabitants were not all Arab back then - they were probably more united by the rulers and being muslim. Saladin lead them militarily against crusaders and became known to the European world.
As amero-euro-centric as Civ is, Saladin is the obvious choice for a leader - emphasis on leader, not ruler, here.
As a side note, I never heard or Harun-ar-Rashid before Civ5. The name is in my language arashid=peanut, perhaps there is a connection, but the name is too silly. Saladin is a much better leader than a peanut.
By this reasoning, no group should be included in Civ unless they have a Leader of some kind recognizable to the European World? In Civ VI as it now stands, that would pretty much leave out Indonesia, Kongo, Nubia, Mapuche, and probably Tomyris except to Classical Scholars. Exactly the opposite: Civ VI, at least, has obviously stretched to find leaders and Civilizations not familiar to the 'average' gamer. I personally happen to think that having done that, it would be a Good Thing to try to base the leader and the Civ on their historical antecedents, rather than throw up a collection of Cartoon leader caricatures.
I think the official course is fine. Gandhi did lead the Indian people to freedom, Saladin did lead Arabian people to victory, Peter did lead Russia towards western world and Gengis did lead Mongolia to conquest...
Also, Civ is pseudohistorical - it builds upon historical facts, but is not tightly constrained. See the quotes in Civ6: " I am fond of pigs..."
We are in semi-agreement: I think they have done a very good job of showing off unusual Civilizations and Leaders, and I applaud that. Where there was no real single political entity for the Civ (as in the Scythians, Arabs, or Greeks) their choices at least have been Iconic where they couldn't, strictly speaking, be Accurate historically.
My point - and I'm a historian, so Of Course it would be my point - is that the closer you can adhere to history, the better the game will be. For one thing, no set of designers no matter how talented can come up with the wildly incredible things that happen in history: the rise of the Arabian Caliphates or the Mongolian transcontinental Khanates are unbelievable as events, and people who achieve what Alexander of Macedon or Temujin did are scarcely more believable as Leaders - except that they were real and so provide templates for Game Characters.
In the end, I think the biggest problem with Civ VI and all the other Civ titles' selection of Leaders has been that having Locked the franchise into a strictly linear (build, build, build, expand, expand, expand) model of City-centric History, the game does a very poor job of representing Civilizations that were not united political entities as well. So, it represents Rome or the United States very well, but Greece, Scythia, Sumer, or any city-state or tribal society very poorly. In all the latter cases, it also guarantees that any leader picked for the game will be a mediocre choice compared to all the potential choices available historically.