Here's why:

* Dissolution: Water can dissolve many minerals, especially those containing calcium, magnesium, and sodium. This process breaks down rocks and creates caves, sinkholes, and other landforms.

* Hydrolysis: Water reacts with minerals in rocks, breaking them down into new minerals. This process is especially important for weathering silicate minerals like feldspar.

* Oxidation: Water can help facilitate the process of oxidation, where minerals react with oxygen. This is particularly important for the weathering of iron-containing minerals.

* Acid Rain: Water can become acidic when it absorbs pollutants like sulfur dioxide and nitrogen oxides from the atmosphere. Acid rain accelerates the weathering process, especially of limestone and marble.

While other factors like oxygen, carbon dioxide, and organic acids contribute to chemical weathering, water plays a crucial role in all of them. It acts as a solvent, a reactant, and a facilitator, making it the most important agent.