It used to be possible though, and now you just slide off no matter the roof because... I still don't know why it was changed.Probably because of shortcuts in world-building: the same reason they can't add flying mounts. There are things that look there but actually aren't. The tops of trees? Intangible. The wall that makes up the trunk only goes so high up. Depending on the tree, you can actually stand on and in it at the same time. A good example is that tree next to the rock near the DC vampire shrine.
When can we have this back? Seriously it was awesome and created so much more fun playing a thief/rogue style character as well as the different environments we could fight around in Cyrodiil.