If you're a young personal at the start of your career, then I agree it could easily make more sense to invest it in yourself (e.g. education, trying out a startup, graduate school).
Every system has friction. You have to concentrate power to overcome friction and get things done. We'd be knocking off the world's problems a lot faster if we could "focus fire" on them. Ten diseases that might each take a decade to cure with 1/10th the donation could be solved half the time or less if each disease got a year of all the donations. But charity is more about signalling your clever pet cause than global optimization of future world-states, so nobody will let go of their second-best (or, realistically, 897th-best) charity.
Do you know of published research that supports this conclusion? I'm honestly curious.