Something to consider is that the Empire is roughly based of Rome.
That doesn't mean originality isn't a good idea.
Now I can more-or-less buy Colovia as a more rugged, Italian landscape. That's fine. The Nibenay river basin, though, could have been heavy rainforest at the same time. That would have been particularly neat since the 1st ed. PGE specifically noted a cultural clash between Colovians and Nibeneans, while in
Oblivion they're kind of all the same. A greater distinction between those parts of Cyrodiil would have been very welcome, and it would have allowed them to keep the medieval feel in areas while also providing more exotic locales elsewhere.
So I'm going with laziness as an explanation. Laziness and dodgy retcons. The potential was clearly there, and there aren't any really convincing reasons why there
couldn't have been a rainforest. Sure, it doesn't make a lot of sense from a climatic perspective, but since when have TES games ever bothered with geographical realism?