I don't think one donates money to Harvard just for the sake of a plaque. I guess there is some sort of indirect benefit to be had after such a donation, like an entry pass for a child, an entry pass for a PhD, or something of that vein?
These billionaires want fame and to be the focus of discussions like this one. They know people will be in awe over such a huge sum of money. That’s what $300M buys you and is so much more interesting than buying a product: attention and fame.