It's interesting to think about from an entrepreneurial perspective, that The Gates Foundation has plenty of capital to "invest" but the companies and programs are often not mature enough to warrant massive grants. There might be a ton of small NGOs doing awesome work all over the world, and they can take small grants (XX,000-XXX,000), but imagine what a nightmare it becomes for them if given the burden of tens of millions of dollars all the sudden. Money solves a lot of problems in the nonprofit world, but sudden cash infusions can often create a lot of bizarre problems.
And on the other side, even if The Gates Foundation gives thousands of six figure grants out, that's not going to make a dent in the overall pool of cash.
Obviously, "staffing up" is time intensive, but I would think they would need an alternative approach if finding worthy organizations becomes difficult.
There may internal pressure to "give money away" but I doubt many have the freedom to start writing checks for ...anything.