philanthropy is just a symptom of policy failure. it's less wasteful than luxury possessions, but those efforts eat up a lot of capital costs just through staffing, etc
Both yachts and philanthropy have most of their money go to staffing costs, and at the level of wealth yacht buying happens at I assume people are spending based solely on their virtues.
The real question though, is why not both? Most people buying super yachts could probably afford to set up a philanthropic fund of some kind.