Partly because it would conflict with YC, partly because I wouldn't be good at it, but most of all because I don't want to. I like dealing with early stage startups, because at that stage the big problem is what to build. I wouldn't be any good at advising founders how to organize an executive team, or how to prepare the company's finances for an IPO.
Incidentally, VCs are not nearly as bad as this comment suggests. The best ones are nice people and genuinely helpful. When they take a long time to make up their minds, it's because they're afraid—not just of the risk of investing in a particular startup, but because that startup has to be the only one of its type they invest in.