That is because of the limitations of the RTW engine. Historically, chariots (and their scythed variants) had an advantage over cavalry because of the stability it provided for the warriors and/or archers on it.

However, this advantage is entirely restricted to level, flat, unobstructed terrain. Hills, forest, even bumpy bits on the ground will quickly negate the advantage of wheels, handing the advantage off to cavalry. Chariots are also less maneuverable than horses, the rider having to control multiple animals pulling a wagon as opposed to just an animal cantering.

The rtw engine does nothing to address any of these limitations. The only penalties hilly terrains bring are universal stamina penalties.