Because emissions are caused by people heating their homes, fueling cars/planes or building stuff for consumption.

With twice as many people (acting similarly) you have twice the emissions, it's as simple as that.

To reduce emissions, you need everyone doing their part. And it is also obviously easier and more effective to tackle high-emitters first (because incentivizing a single US family to have their second car be a bit smaller and electric is obviously less burdensome than banning 3 Indian families from heating their homes in winter...)