Because China's population is 1.4 billion people.
What's more important is the rate of change. Every country goes along a curve where emissions increase, peak, and then drop. China was on the up slope as it industrialized, and is now starting on the down slope. The peak was far, far below where the US and Europe peaked per capita, the only reasonable measure unless you think their One Child policy was a good idea.
The rate at which China is installing renewables and storage is staggering. Just the newly installed renewable capacity from 2025 produced as much electrical energy as the whole of Germany consumed that year. Just the new stuff, and this year they are on track for it to be even more. They are adding more than a Germany sized amount of renewable generation to the grid, PER YEAR, and accelerating.
If Europe and the US were doing that too we would be in a much better position regarding climate change. It would doubtless be a huge boon for those economies as well.