>>105 et al
"these certain races are still living in primitive societies or in poverty and in third world countries"
That's actually not intelligence, that's resources. No, not gold or silver or any of that junk (in this case), there's at least 4 (off the top of my head) things a growing empire needs:
1:
Crop plants
By that I mean something that can be cultivated on a large scale, to feed huge cities (10k+ might not sound like much now, but we're talking stone and bronze age tech). I'm not aware of all that many such plants coming out of, say, Sub-Saharan Africa.
Remember, civs have perished for ignoring this one. Yes, really!
2:
Livestock and
pack animals
Once again, I'm not aware of all that many usable animals from SSA. Water buffalos and zebras are too temperamental (cows and horses are zombies by comparison. They're also imported).
Livestock is needed for meat, leather, fur, wool, etc.
Pack animals and riding animals are needed for at least two things: military and trade. Anyone who's seen too many movies like Arthur will see the need for cavalry, but let's not underestimate the need for trade.
Example: The Ghana Empire (that the current Republic of Ghana took their name after) started as a small(ish) kingdom, and didn't really take off until Arabs brought camels and camel caravans. Suddenly they could export salt (expensive in those days), gold, ivory etc, over long enough distances and in big enough amounts to grow big and get rich (enough to attract Almovarid invaders).
3:
Building materials
Seriously, a single-story mud hut can only house so many people, you can only administer so much from one. Also, there's stuff like waterproofing the roof.
The very least you can do with a huge building with a smooth surface, is to duly impress subjects and visitors. Then there's housing the bureaucracy an empire will inevitably need, not to mention how much more people you can put into an area with multi-story buildings than one with mud huts and tents.
4:
No parasites
Didn't see that one coming, did you?
I'm talking about stuff like mosquitos (malaria) and tsetse flies (sleeping sickness). Tropical areas, especially moist ones like rivers and swamps, are crawling with the stuff. More than just itch for a few days, they can kill both you and the horse you rode in on. Literally.
I sair "at least" and "off the top of my head". That means, by definition, that there's bound to be more. Like, say, the advantage of the main land mass being oriented east-west (like Eurasia) rather than north-south (like America and Africa), as technology, livestock etc will be equally available over a larger area once they pop up in one spot. (hint: try putting up an igloo in Nevada...) (hint2: the horse was tamed in central Asia first. Suddenly "everyone" in Eurasia rode horses.)