Here's why:

* Structure: Unlike crystalline solids with a highly ordered arrangement of atoms, glass has a disordered, random structure.

* Flow: While it's true that glass can flow over very long periods, this flow is extremely slow at room temperature and practically imperceptible.

* Shape: Glass retains its shape unless subjected to significant external forces. This is a defining characteristic of a solid.

The "Liquid" Argument:

Some people consider glass a "supercooled liquid" because its molecules are in a disordered state similar to a liquid. However, the fact that it retains its shape over long periods distinguishes it from true liquids.

In conclusion: While glass exhibits some liquid-like properties over extremely long timescales, it's more accurately classified as an amorphous solid at room temperature.