Because that's the level at which you'll have more impact, and at which you can more accurately gauge the impact you're having.
Some international organizations will report impressive results, and it's a reasonable argument to say you should leave the do-gooding to the professionals, take advantage of economies of scale, etc.
No. First of all, Be careful when optimizing something you can easily measure—is the impact those orgs are claiming truly the thing that needs to change in the world?
Also, Consider side effects—does the presence of those organizations help prop up exploitative leadership that otherwise would have to answer to their citizens? (The Dictator's Handbook is a great read on this topic.)
And what are the externalities associated with a decision to maximize income in order to donate to these causes? What do you do at your job, and what's the impact of it? Might you do more good if you Keep cost of living low, Optimize for discretionary time, and give more of your energy to the people around you?
Even issues that seem to require global coordination, like climate change, will require actual humans (including you) to figure out how to produce food and survive the elements where you live. What's the plan?
It's true that Every place on earth has unsafe levels of plastic in the rainwater, but your town has to find a way to provide everyone with clean drinking water. You could probably help with that.
Keep your focus local, most of the time. Vote in national elections every couple years, but organize locally on a weekly basis. Network and commiserate with like-minded people around the world, but apply what you learn from them in your own community.