People want other people not to pollute, but don’t always have an incentive (a good reason) not to pollute themselves.
It is hard making incentives to stop people from polluting.
Businesses may pollute because their competitors pollute, and they would make less money (or go out of business) if they didn’t pollute.
Some countries may not pass anti-pollution laws because they don’t want to hurt the businesses in their country. Still, this pollution might affect the whole world.
Some people want to protect the environment, but buy products from polluting factories or polluting countries anyway because the products are not expensive.