Because he's a rich old git who has better things to do than do something nice for the people who got him into the position of being a rich old git?Why would you think he would be complaining about getting so much fan mail?
And if people who write more than once or twice are a problem then there is always the Ian McKellen or George Foreman approach. Sign 1 item per person, or keep a list of who has written to you previously. And if your so worried about people flogging your autographs on ebay then personalize everything and sign every request, if everything is personalized and your autograph is ten a penny it will be worthless. problem solved. Which i expect isn't the case anyway, more the rich old git syndrome.
He was definatly less than nice and apologetic about it anyway from the video.
The Beatles may have been cool in the day but the last 2 are moody old farts now. Don't waste your time on them.