How? What do they have to offer?
Very few people 'make millions' from delivering speeches. We're talking the likes of Clinton, Obama and high profile business people who many of us have never heard of. A handful of people who bring prestige and bragging rights to an organisation. You can boast about that time you had Obama speak at your gala dinner,or point to that photo of you shaking hands with Bill Clinton at your convention last year.
How does the exiled 5th in line to the British throne or his wife who once played a paralegal on 'Suits' come even close to that level of prestige?