I'm sure Bethesda would have loved to make Cyrodiil a jungle, how ever the game engine is very limited in rendering it properly, without melting most computers. I doubt even high end ones could do it properly.
Then they could have picked a different game engine, or invested some time to make a low-fps-killing alternative to how trees could be rendered. Heck, Shadow of the Colossus could render stuff better and at a greater frame-rate that Oblivion's engine could if it attempted the same feat. And SoC was for the PS2's hardware!
If the coding is the problem, then
do it better. Beth didn't have to stick with Gamebryo. MW is the first time they used it. They could have said: "screw it, we can't even prevent the rain from going through roofs in exteriors" and picked something else.