Answer:
Erosion breaks existing rocks and turns them into sediments. The sediments can be formed into rocks by the chemical reactions of dissolved seashells ( Calcium Carbonate.) This reaction can form sediments into rocks very quickly like the formation of concrete ( same chemical reaction). Erosion only happens on the earth's surface.
As erosion only happens on the earth's surface, sedimentary rocks are only formed on the earth's surface.
In other words:
Sedimentary rocks only found on or close to Earth's surface because sedimentary rocks are made of materials deposited in layers at the Earth's surface.