Why does glass fracture more easily than steel when stress is concentrated in one spot?

Glass fractures more easily than steel when stress is concentrated in one spot primarily because of a fundamental difference in ductility.

Ductility is the capacity of a material to deform permanently in response to stress, allowing it to absorb and redistribute local stress concentrations. Most common steels are highly ductile; when subjected to stress, they can undergo plastic deformation—such as "necking"—which prevents the immediate propagation of cracks.1

In contrast, glass is a brittle material that lacks this ability to yield or bend. Because glass lacks the dislocations and grain boundaries found in crystalline metals, it cannot accommodate local stress concentrations. Furthermore, commercial glass often contains microscopic surface flaws that act as stress-concentrating sites. At the tip of such a flaw, the effective stress can be 100 to 1,000 times greater than the applied stress, causing the material to fail suddenly and completely rather than deforming.23