It's known that money has a powerful effect on human psyche, and I strongly doubt that pouring say a million dollar to a charity that is used to manage ~10K$ donations will suddenly multiply its efficency hundredfolds. More likely, that money will be wasted. Or given to local warlords. Or used to pay higher salaries, etc.

Why would you give to charities past the point where they have room for more funding?

The unreasonable assumption here is that there's a direct path from money to utility for other people, as if charities were engines of perfect efficiency that burns money and churns out utility for poor people. Extremely unlikely, even for charities listed top tier on GiveWell.

Life is full of uncertainty, and there's nothing wrong with that. If you're saving lives on expectation, it's worth doing.

I agree that investing in R&D can be a better way to improve the world than donating to traditional charities.