Here's why:

* Erosion: Caves are most commonly formed by the erosive power of water, often acidic water containing dissolved carbon dioxide. This acidic water reacts with the surrounding rock, such as limestone, dissolving it over time and creating cavities.

* Deposition: While deposition can play a role in the formation of some cave features, it's not the primary force behind cave creation. For example, stalactites and stalagmites are formed by the deposition of minerals from dripping water, but they are features *within* caves, not the caves themselves.

Key takeaway: Caves are formed by the *removal* of rock through erosion, not the *addition* of material through deposition.