Essentially because I think Keynes (and most economists of his era) were right in their basic assumptions. If you made the average person in the 1920 10X richer, they'd have behaved much like the "leisure class" of the time.
If you ask the average person "why don't you work 65% less and chill?" most people would look at you funny. It's not a choice. I have rent, etc. A solution that relies exclusively on choice fails the smell test.
I have my own ideas about the solution, but that's besides the point. My point was that Keynes, not knowing the future, gave a more intellectually honest projection of what the theories predict than any post facto economist would have.
note: no reason to downvote a respectful, on topic comment. Asking "why not" is the whole point of discussions.