Telvanni culture is based around the application of magic. Their houses are made from the ground up and require levitation to navigate. Redoran culture is based around honor and spartan simplicity. Their houses are the armored carapaces of deceased (mayhaps killed by the Redoran?) bugs, and serve not only as metaphor of knighthood but also as a show of their love of the practical. Hlaalu culture is based around trade and politics, adapt at the reinvention of their identity. Their houses therefore reflect more western principals.
Not everyone belongs to a Great House, even in Sadrith Mora. Most are just commoners or traders who live in whatever variety of housing is available, and they don't make the same distinctions. Most of the distinctions are either by local events varying by location, or politics by faction, such as a Great House.
The Colovian and Nibenese peoples do not share these distinctions however. There is very little (if any) distinction in their value systems and in their behaviors. The style of dress is homogenous across the province and architecture is divided by city, not by culture. Much more could have been done to establish the Niben and Colovian as independent zones.
Every town has a variation in culture, instead of only two big cultures. Look closely and it's clearly there in each town. And either way the only real source that it contradicts is over 400 years old and at the start of the era, whereas now is the end of the era. But even then there are places here and there that make distinctions between colovian and nibenese.
But on a somewhat less related note, while the politics in Cyrodiil certainly are lacking, it would be unreasonable to expect the politics of the center of the Empire to be as conflict-filled as in Morrowind, where assassination and private war has been commonplace for millennia.