Has philanthropy grown comfortable with mediocrity?

I was just reading an interesting article in Forbes about the philanthropy of billionaires and this quote from the article hit me over the head like a ton of grant proposals:

It's worth remembering that no philanthropist has solved a worldwide problem since Carnegie brought universal access for the poor to books via libraries (1883-1929), and Rockefeller used his billions to fund the research that would lead to the eradication of polio (1952).

Gates' ambition is on a similar scale. He wants to eradicate the 20 leading diseases in the world during his (or his wife's) lifetime.

I am excited about Gates' vision and I think he has the right pieces in place to accomplish that vision (lots of money, staff experts, and a results oriented funding model) but I am very concerned that his is one of the few foundations that has such a bold vision and a plan to accomplish it. Many foundations either lack the nerve to announce that they will solve a problem, these foundations use statements like "helping people in poverty" or "reducing disparities" or the foundations that have a vision like "ending homelessness" do not have the scale or partners needed to actually make that happen.

My question for you is why is it that the professionalization of philanthropy and the growth in wealth in the "developed" world has not led to a true global solution since polio?