You will be shown an excerpt from a computer science scientific research paper, followed by a question about the paper. Please answer the question. Do not include any text in your response other than the answer.

Context: Visual SLAM can be performed by using just a monocular camera, which is the cheapest and smallest sensor setup.However as depth is not observable from just one camera, the scale of the map andestimated trajectory is unknown. In addition the system bootstrapping require multi-view or filtering techniques to produce an initial map as it cannot be triangulated from the veryfirst frame. Last but not least, monocular SLAM suffers from scale drift and may fail if performing pure rotations in exploration. By using a stereo oran RGB-D camera all these issues are solved and allows for the most reliable Visual SLAM solutions.

Question: Why are pure rotations hard to track for monocular SLAM?
Pure rotations hard to track for monocular SLAM because  depth is not observable from just one camera, the scale of the map and estimated trajectory is unknown.