Because the thermal interface material, either a "pad" or paste, that was applied between the GPU/CPU and it's heatsink, dries out over time and either stops working as well to transfer heat to the heat sink or stops working at all, if a gap is formed by the TIM drying out and shrinking to the point where the heatsink no longer touches the surface of the die surface.
Also, electronics wear out. Most laptops don't last even five years much less nine or ten like yours. When you factor in that it's a very low end GPU to start with, so it comes with minimal cooling capability, plus you use it for gaming, not surprising at all really.
Still, it could simply be a software issue, like a driver, but it's doubtful.