Because your family is rich enough for you to remain out of the job market all those extra years. I wouldn't be surprised if the PhD becomes a new indicator of wealth and power once everyone realizes how uneconomical it is.
I knew fewer folks in the humanities, but you might be right there: some were certainly doing this on a lark.
A PhD is financially viable for anyone who could make ends meet while working at Best Buy. Most people working on a PhD have no family or mortgage.