Pigswill: There may be a bit of that but .... Was the Mediterranean Sea rich in fish historically? Yes. Did Italy, France, Spain, England, and Greece all have access to iron within their borders in roughly the geographical area depicted in E18? Yes. Were there horses historically in Europe? Yes. Did it have more wine than other parts of the Old World? Yes. Did France and England have coal mines? Yes. Is the North Sea rich in seafood? Yes. Was there wheat fields in France, England, Poland, and northern Italy? Yes. Was there stone and marble available in the places depicted in E18? Absolutely. I believe there are more ancient marble structures in Europe than anywhere else and the marble was not imported from afar.
Does Western Africa have any of that? Answer is no. They have elephants, rivers, and salt which was where their wealth came from during their Golden Age. In modern times, oil is located there .. just as depicted in E18.
I think it's a case where the Civ4's simplified model falls a little short.
Barring a few rare circumstances, map tiles don't change; a grassland stays a grassland, a desert stays a desert, wheat bonuses stay where they start. But in reality? 15000 years ago, the whole swathe of Northern Africa was pretty much uninhabitable desert. Then the whole region starts getting more rainfall and holding onto more water, greens up, and 6kya (when the Earth18 map nominally starts) most of it is rolling green savannah, eminently well-suited for human life and agriculture. In Civ4 terms, grassland terrain with the occasional plains. Today? Most of it is desert again.
In 500 BC, France and Spain didn't have grapes. Carthage was well known for its grapes, a product of human cultivation in the region. Then Rome takes center stage; Carthage undergoes a dramatic fall from grace, while Roman expansion into Western Europe brings grape cultivation with them. The France region has wine in the Earth18 map because France is well-known for wine today, and people playing as the French civilization on the Earth18 map would object if they didn't get wine in their starting region.
Wheat, in the form that we know it today, isn't a naturally occurring plant anywhere. It's the product of thousands upon thousands of years of cultivation and selective breeding to produce strains well-suited for the climates that people were living and trying to grow it in.
And then there's the way that Civ4 gates resource prospecting behind specific techs. If you have Iron Working, you see all iron deposits. Everywhere you've explored, instantly. If you don't have that tech, you don't see any of them. In reality? Sometimes the iron is less easily accessible, either buried deeper or in a more inhospitable region - it's not until the population has grown in the region and lived there for a while that they become aware. Sometimes it's accessible, but there wasn't a lot of historical motivation to use the iron there - regions not easily linked to trade networks and with limited local demand. Mali is actually a great example for this; on Earth18, the only iron they can get is in modern-day Morocco. On actual real-life Earth, the foothills of Guinea have one of the largest untapped iron deposits in the world (on the Earth18 map, this would probably be analogous to an iron resource 3S1W of the Mali starting settler, touching corners with the elephant).