Why do many plastic objects become brittle after years in outdoor sunlight?

Answer: Sunlight triggers chemical damage to the long chains in the plastic

Cool nights freeze the plastic into a permanently harder state — Cooling can make some plastics stiffer, but it does not normally lock them permanently into a frozen state. Sunlight-driven chemical aging can leave a plastic brittle even after it warms up.

Sunlight triggers chemical damage to the long chains in the plastic ✓ — Ultraviolet light can trigger reactions, often involving oxygen, that damage polymer chains. Chain breakage and other structural changes can make the material less able to stretch before cracking; different plastics resist this damage differently.

Sunlight dries out water that normally keeps the plastic flexible — Outdoor aging is not generally explained by ordinary water evaporating from plastic. Some plastics absorb moisture, but long-term sunlight exposure can change the material chemically rather than just dry it.