Why are fossil shells on top of the mountains?

Because shells on mountains are easilly explained by uplift of the land

This phenomenon can be explained by plate tectonics and it's results. The theory of plate tectonics states that the earth's crust is divided into plates and these plates float on the semi molten mantle below. This resulting movement, caused by convection currents, cause plates to come together, move apart and slide past each other. Plates come together at a convergent boundary. As the plates approach each other sediments in ancient ocean basins will get folded and thrusted upwards. Overtime this movement forms extensive mountain ranges with the ancient ocean sediments filled with the remains of marine organisms. This is why one can observe marine fossils atop many mountain ranges.