Solids are not called fluids because they lack the ability to flow and take the shape of their container. Fluids, on the other hand, are substances that can flow and readily conform to the shape of their container. This fundamental difference arises from the distinct molecular arrangements and intermolecular forces present in solids and fluids.

In solids, the particles are tightly packed and held together by strong intermolecular forces, such as covalent bonds or ionic bonds. These forces create a rigid structure, preventing the particles from moving freely past each other. As a result, solids maintain a definite shape and volume and do not flow.

In contrast, fluids are composed of particles that are relatively far apart and weakly bonded by intermolecular forces such as van der Waals forces or hydrogen bonds. These forces allow the particles to move freely and slide past each other, enabling fluids to take the shape of their container and exhibit fluidity.

Therefore, the inability of solids to flow due to their strong intermolecular forces distinguishes them from fluids. Solids maintain a fixed shape and volume, while fluids adapt to the shape of their container and readily flow.