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

Because it's a very uncertain point

We have found very little helpful research or analysis on the "room for more funding" question, and feel that it is largely neglected in public discussions outside of GiveWell.

Also:

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

Yes, unless you're neglecting a better way readily available. In which case you're paradoxically killing people on expectation.