[MperorM]

The best steelman I can come up with is that it's very difficult to give away that many billions effectively.

Give one billion to every charity on givewell's list, and now those have more money than can effectively spend in the foreseeable future. Yet you wil have spent nowhere close to your entire fortune.

While I don't understand why billionaires haven't done so yet, I can understand why it may make sense to be patient and search for great charitable opportunities.