Because there is no regulation or enforcement, so the manufacturing company's throw their waste in the rivers. If you want sources, do your own research.
1.5Billion people leave a big foot print of garbage waste and toxic chemicals China has a lot of illegal dumping into the water and into the land this causes a lot of pollution and the air is easy to explain the stuff that come outs of the back of your car causes the smog also coal burning power plants and many other sources.