Because, as I wrote, you could prefer a more consistent look or the preferred fonts are not rendered very good on different platforms (e.g. Helvetica).
This is strange logic, at least when considering the font stack you recommend earlier. If cross browser consistency is important to you then the most desirable font (for you) is one that renders constantly across all browsers and that's the font that belongs at the top of the stack. To say that your most desired font doesn't belong at the top of the stack is misleading.
Why bother including the much less common and much less desirable fonts in the stack at all? There's virtually no chance that someone's not going to have Arial, "Arial Unicode MS", Helvetica, "Helvetica Neue", HelveticaNeue, "Nimbus Sans L", and FreeSans installed but will have Liberation Sans.