Here's why:

* Erosion and Transportation: Sedimentary rocks are formed from the breakdown of existing rocks (igneous, metamorphic, or even other sedimentary rocks). This breakdown occurs through weathering (wind, rain, ice, etc.), and the resulting sediments are then transported by wind, ice, or most commonly, water.

* Deposition and Burial: Water is the most efficient medium for carrying and depositing large quantities of sediment. Rivers, lakes, oceans, and even groundwater systems all play a role in depositing sediment. Once buried, the sediment layers are compressed and cemented over time, forming sedimentary rocks.

However, it's important to note that some sedimentary rocks are formed in terrestrial environments:

* Desert Sandstones: These rocks form from wind-blown sand, deposited in desert environments.

* Glacial Sediments: Glaciers can carve out valleys and transport large amounts of sediment, forming till, moraines, and other glacial deposits.

* Loess Deposits: Fine-grained sediment carried by wind and deposited in thick layers, often found in areas downwind of deserts.

While water is the primary agent in the formation of most sedimentary rocks, terrestrial environments also play a significant role in the sedimentary rock record.