There is an entry on conditional spawns in the civilopedia.
You're right that there are many cases where historical plausibility would dictate a spawn should be historical (making all medieval European civs conditional is a popular suggestion that comes up every couple of months).
It should be obvious why I am not doing that, though. We know that America came to exist because English colonies existed in North America. We do not know what an independent North American colony from Dutch/Spanish/French ... settlements would look like (much less if such a thing would even come into existence), so generic Anglo-cultured America has to take the place regardless. And nobody knows what would have happened if nobody had settled North America, so I have no civ on hand for any other situation, and leaving NA empty is undesirable. I have only history to go on, anything else is speculation beyond a certain point, and has no place in this type of mod.
Applying the same reasoning to European civs is possible but the adverse consequences are even worse, because they shape the situation on most of the rest of the map.
You will notice that all conditional spawns affect areas where multiple civ's territories overlap, and won't be empty when they don't spawn (contrast America or Euros). As stated above, one of the reasons for their conditional spawn is that their existence doesn't screw over the game of those who are already there.
Another reason is that this kind of game relies on a combination of familiarity and unpredictability, and wouldn't work with just one of the two. Unpredictability makes the game interesting - if everything would always develop in the same way (what some people think a "historical" mod should be like), it would quickly become boring. Also, as a player you need to be able to influence the world around you and observe the consequences of these actions.
On the other hand, to make it feel like a historical mod, we need some familiarity. If we play say as Turkey, we want to play in an environment that is approximately similar to the historical environment of actual Turkey. It might be cool to spawn and find Europe controlled by a Celtic empire while Russia has founded Islam after Buddhist Persia has suppressed the Arabs or something. But if your goal is to recreate the historical Ottoman Empire, you kinda want to be Muslim and fight Christian Byzantines and Muslim Arabs. So the game requires some railroading, and civ spawns provide much of that (the other important factors are collapses and settler maps).