Even they didn't try to take on all the problems in the world. They helped a subset of people that they cared about with particular fairly well-defined problems.

Yes, that is how adults help in real life. In science we chop off little sub-sub-problems we think we can address to do our part to address larger questions whose answers no one person will ever find alone, and thus end up doing enormous work on the shoulders of giants. It works roughly the same in activism.