Why Isn't there an Option to Donate Ad Word Revenue to Charity?

There are a number of people who are donating the proceeds of their blogs to charity. There are also a number of dedicated sites such as Squidoo who allow for donation of ad revenue to charity.

So why not Google? It would easy for them to do, and simply adding the option within the ad sense administration tool would mean saving bloggers from having to manually cut a cheque, track revenue, etc.

Just adding that as an option would help charities raise some funds...