Why are sedimentary rocks only found on or close to Earth's surface?

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.