What is the reasons why people are throwing wastes in the rivers and huge water bodies?
Lack of proper regulation, harder to pinpoint the source
Large of water exist where nobody technically owns the space. Like, no one owns the ocean, technically. Also in a larger water source it can be harder to pinpoint the exact point in which the pollution has occurred (if it isn't just dumping). It takes time and resources to regulate water use for large bodies of water, a responsibility no one group often can do.
There are different countries that share ocean and some rivers, and within these countries different agencies that have to cooperate to hold individuals and companies responsible. (It's also hard to take care of your stretch of water if your neighbors don't.)
So it results in open unregulated space with not many governing bodies to answer to. It's also very convenient.