Says who? They can (ethically) invest it and fund programs off a 5%-8% or better return. They can find new things to do. They can donate some of it into health research that is currently under-funded.
And with such a sum of money they would surely have to hire staff to work all that out. Can thy do that?
I’ve tried volunteering at certain orgs before, I filled out forms and literally they rejected me because they had no more staff to organise and oversee more volunteers.
If your solution is just invest it, well, the Gates Foundation may as well hang on to it (you think you can do better job than Buffet?) and setup a system to dole it out.
If the org has to find new uses for it, surely the Gates Foundation is in a better position to get that done?