Those genasi pictures you posted weren't very "human" looking. Still the flaming hair and green skin. Still very "out there" and certainly not "human with a touch of otherness".
And remember, there were humans on Abeir. Lots of them. The majority. And probably elves and Dwarves and such. Same standard races. We don't know how the dragons created the dragonborn. Make up your own reasons why they are humanoid.
I agree that they were still more obvious than I like, and also not entirely in line with the description in the text. But they aren't nearly as obvious as the ones pictured nowadays.
It's not that the dragonborn is not that they lived with other humanoids like humans and dwarves. While civilizations can be impacted by other civilizations, evolution is not. Dragonborn are designed like a human in a dragon-person costume, in both appearance and culture. I find that uninspiring, and also wonder what's the point?
The point is that they want a game where every PC race is a roughly humanoid creature, of small or medium build, that will fit the same armor and can use the same weapons as other humanoids. In our world, during medieval times, there was a wide range of civilized cultures. While the middle ages of armored troops armed with swords was prevalent in Europe, with variations in Asia, vastly different (usually considered more primitive) cultures existed in Australia, other parts of Asia, Africa, and the Americas. Our needs and desires are one thing, why wouldn't the dragonborn value something different? Why would they build the same types of cities and towns, instead of something entirely unique?
If I were to design a race related to dragons, it would be quite different. They'd probably have wings and tails, to start. And I still wouldn't just dump them into the Forgotten Realms. I wouldn't design them with a singular point of view for the entire race either. Like humans, elves, dwarves, etc. they would have subraces and complex societies, and be one of the dominant races, if not the dominant race in their world. If they were enslaved by dragons, then they most likely would develop underground cities, with tunnels too small for the dragons. Their combat tactics would be centered on attacking from many different directions at once, from cave mouths with stone doors about 20-feet into the cave that could be shut quickly to protect against the dragon's breath weapons. Ranged weapons and magic would be preferred where possible, and melee combat would most likely center on their natural weapons and their own breath weapons.
Being partially quadrupedal they'd be good climbers, and I'm not sure they would develop the same types of weapons as humans because they would be accomplished hunters without having to resort to weapons. Humans require weapons to bring down large prey. A few dragonborn, particularly with wings, would be able to bring down prey with little difficulty in a group. As a result, I'm not sure they'd ever need to develop animal husbandry, or raise herds of animals, although it's possible that if civilization grew that they might.
On the other hand, as able hunters, and being well suited for self defense as well as hunting, they may not have needed to gather together in groups to take advantage of the benefits of farming (especially if they are carnivores) and protection that a larger settlement provides. Without dragon oppressors they might be more likely to live as nomads and roving hunters within their extended family or clan, with fierce rivalries over territories and prey among clans.
Nomadic hunting clans are also not conducive to mining activities. So metal weapons, particularly steel which would require the evolution of advanced mining and metallurgy techniques, may not be part of their culture at all. Being strong with natural weapons, a show of strength with their natural weapons might also be considered more honorable.
If they primarily live a nomadic life following migratory prey, they may not have to develop a sophisticated language, particularly written. Trade as we know it, or currency, with the need for minted coins or some equivalent may not have a need to develop either.
Humans, elves, dwarves, halflings, and similar races have little natural protection or weapons. They rely on numbers for safety, clothing and shelter in hostile environments, agriculture to support civilization, and sophisticated languages to foster the trade in service, work, and goods that life in an organized civilization requires. In addition, their young are helpless for an extended period of time, and need the care of a family to survive. While it's entirely possible that a race of draconic humanoids could evolve in this fashion, I think it's unlikely. There are too many differences in the physiology and biology to expect that they would grow along the same trajectory.
There are so many places they could have gone. And yes, there are some contradictions above, this is just a stream-of-thought look at what could be. But this all depends on them having a well developed world of their own.
Ilbranteloth