And yes, they also started building river interceptors as that is more bang per buck. They still seem to think they can clean the existing stuff economically too.
It would be interesting to see $ spent per tonne of garbage removed from the rivers vs ocean. Like you said, far more economical to focus on the source. Given no project has unlimited funds, why spend resources on the"garbage patch"? I suspect that they still work on the ocean part because:
1. Is better for fundraising;
2. Is a fun engineering challenge;
3. fun to work on big boats;
4. Helps with recruitment;
Check their Interceptors.
https://theoceancleanup.com/rivers/
Also, the ocean garbage does need cleaning up too