The composition of sewage is more predictable and a lot of the contaminants are organic, so bacteria can do a lot of the cleaning for us. If you go to a treatment plant, you can see that most of the process is just letting the water sit in big aerated tanks while the bacteria eat the poop.

The seawater has been sloshing around for millions of years and is full of minerals and things bacteria don't eat so you need to either distill it or pump it through a lot of filters.