Why are there no trolls or wargs?

Wargs can't climb.

Traversal and climbing is an essential part of gameplay in Shadow of Mordor, but we know that wargs are not effective climbers--otherwise a group of wargs would have made a meal out of Thorin and Company during their escape from the goblins of the Misty Mountains. Monolith needed a climbing mount, otherwise players would feel limited when riding their mount instead of feeling empowered. A new creature had to be created.

Simply put, this is an example of the existing fiction not being compatible with the gameplay needs of the product.

Right, but that's still what this thread is about.

@krullban: it has been stated that the graug and caragor are completely different beasts. The game devs stated that "Graugs could eat trolls for breakfast" and "wargs are wolves, caragors are lions"

I didn't understood the difference either. The games are very, very loose with the LotR myths, so it is not like they did it to distance themselves from the source material.