I can't speak for them, but I expect it's something like this: One can make more money, do more good, have a more fun career, and have more freedom in where one lives by dropping out than by going into academia. And having a PhD when hunting for non-academic jobs is not worth spending several years as a grad student doing what one feels is non-valuable work for little pay.

You'd have to speak to someone who successfully dropped out to get more details; and of course even if all their judgments are correct, they may not be correct for you.