Maybe because the only forms of death-defying magic known to wizardry are things like Horcruxing that would only be done by the Bad Guys, so that the idea of defying death is seen as characteristic of Bad Guys.
Well, I haven't read canon, but I think Flamel is portrayed as a Good Guy whose elixir-of-life-producing-rock is sought by Dark Wizards. (And I think he'd used it on himself and remained a Good Guy.)
I'm guessing in MoR there should be no (actually working to significantly prolong life) philosopher's stone, as not using that more widely would be altogether too crazy.
Update: This post has also been superseded - new comments belong in the latest thread.
The second thread has now also exceeded 500 comments, so after 42 chapters of MoR it's time for a new thread.
From the first thread: