Something to consider is that the Empire is roughly based of Rome. Can you imagine Roman soldiers wandering around in Jungle? To be honest, the flora that they selected for Oblivion felt more suitable to me, despite what the PGE says.
Personally, when one only looks at the written lore (so not in-game armour, etc.) I have simply no idea where anyone gets the idea Imperials were based on the Roman Empire. There were plenty of other important Empires in human history, and if I had to pick one that the Empire from TES was based on, it's the Chinese Empire. Sure, there are some political matters similar to Rome (such as the mad Emperor, Pelagius), and some historical events similar to events that happened during the Roman period, but as far as culture goes...
Also, remember, from what I remember, Cyrodiil isn't covered with jungle, it's covered with rainforest. Two different things, because:
Jungle = A large, undeveloped, humid forest that is home to many wild plants and animals.
Rainforest = A forest in a climate with high annual rainfall and no dry season.
Of course, I may have recalled incorrectly.