The problem is that the people who are doing it to become a professor are playing a game that is stacked against them. And if/when you lose that game you're well behind your peers in opportunity cost (see my other post).

I know several brilliant scientists (postdocs and PhD students) from my time at Princeton who had / are having a very hard time finding an academic job. At some point even the purest motivations will succumb to feeling used in these circumstances.