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;