Four thousand six hundred and ninety-one irradiated haggis

I suspect because he doesn't appear to be particularly malicious. He may say all sorts of horribly offensive things, but it's difficult to imagine Prince-Philip-The-Public-Persona ever actually wanting to cause anyone any hurt - most of his most awful comments are him trying to put people at their ease.
Of course for all I know he might be an absolute monster in real life, but in a weird way making jokes about 'cannibals' or 'slitty eyes' or whatever is *almost* 'celebrating diversity' in its own perverse way - he seems to *like* these stereotypes existing, and not think badly of the people themselves for being (in his eyes) ridiculous. I think he's *trying* (or trying to appear like he's trying) to laugh with, rather than at, people, and just gets it hideously wrong.