Here's why:

* Clastic rocks are formed from fragments of other rocks. Flint is formed from the accumulation of the microscopic silica skeletons of single-celled organisms called diatoms and radiolarians. These skeletons are essentially tiny fragments of rock material.

* Organic rocks are formed from the remains of living organisms, like coal or limestone. While flint does originate from living organisms, the rock itself is formed from the mineral silica, which is a component of the Earth's crust, not from the direct organic remains of the organisms.

Therefore, while flint's formation involves biological processes, the final rock is a result of the accumulation and cementation of clastic material.