It's not just that - even when we're talking Warren Buffett here, the scale of the US govt is totally different. Buffett's entire fortune ($50B according to Wikipedia) would only cover a few weeks' worth of borrowing and then we're right back where we started only Warren Buffett has no more fortune. It's silly to say we should expect him to do that.
What he's saying by advocating tax raises is "I will pay more if I know that everyone else will also pay more." He's not interested in paying $XM to reduce the debt by $XM, but he would happily pay a personal cost of $XM if it meant reducing the debt by $XB.
(This is called "collective action," and it's also Groupon's business model.)