The source of the trouble is that the uppercase G of their logo font is too lightweight to look good by itself. The Yahoo Y looks pretty good, but the Google G is all empty space. And it's probably not a font where you could get away with cranking up the weight very much.
Basically, they've been hosed by bad design choices they made early on, and which they now (rightly or wrongly) consider to be part of the company's identity.