Where do you draw the line. European civs are incredibly similar in style and culture as well.
It would be ridiculous to say Serbia shouldn't be in civ because they're too similar to [insert inner European, Slavic, Christian state here].
I'm sad Portugal is missing at launch yet again, even if it is similar to [European colonial naval power that spread its language to new regions and made a lot of money off the exploitation of people and places]. I don't care that Portugal is similar, they deserve to be in civ.
They picked "Abassids" instead of "Arabia". It would be fun to eventually have all the other main caliphates as well. Would they be similar? Sure. Are they similar irl? If course. Can the devs come up with fun ways to differentiate them? I'm sure they can.
First of all, as stated by others already, there is no 'line' to be drawn: in an ideal gaming world, every group, culture, politie, and Civ that has ever existed or can be proven to have existed, even if only in a writer's mind, would be included.
That, of course, is not possible.
My personal criteria is that unless there is some Playable game mechanic, bonus, activity, Unique, or some other in-game criteria that can be applied to differentiate one Civ/culture from another, they do not need to be included until we run out of all other alternatives. Some people might enjoy playing a game in which all their opponents are Achaemenid Persians by other names, but for most of us, it is the variety of opponents and necessary responses to that variety that is part of the game's attraction.
So, in this particular instance, Medes and (Achaemenid) Persians are so similar that it is very difficult to find in-game capabilities that would separate them into distinct Civs.
Likewise, as I've posted before, the central Asian pastoral 'Civs' are so similar in may of their basic aspects that it is hard to differentiate them: Scythians, Sarmatians, Xiongnu, Huns, Kushans - it is not impossible, but is is not easy either, to find distinctive attributes to assign to them without invoking Emblematic Leaders (which Civ VII no longer does) like Attila, Tomyris, or Modu Chanyu.
Your argument about Portugal 'deserving' to be included seems to me to be based on the impact that a group had on history, which in Portugal's case was considerable (especially in relation to the size and population of the original country) BUT I would point out that it was also very limited in Time. Portugal's rise to explorational power, founding of colonies and trade routes and status as a major naval/trading nation lasted, basically, less than 200 years - a fraction of a single Age in Civ VII. That doesn't preclude them from the game by any means, but I suggest it places them slightly lower on the selection list using the 'Historical Importance' criteria than, say, Spain, England or France, all of whom had far more influence for far longer
Given the limitations in number of Civs that can be included, even stretching to include years of DLCs, compared to the total number of possibilities, choices have to be made. I believe those choices should be made in favor of Civs and groups that bring Something Different to the game rather than repeat what other Civs already have brought to the gaming table.
And although I agree that Portugal deserves a seat at the table, I would also argue that given Spain's importance as a colonizing, initial trade route and exploration Civ, both of them do not need to be included from the beginning when there are so many other areas, time frames, peoples, cultures and Civs to be represented, for Variety's sake if nothing else.