No-one quite says that working hard is stupid (well, except the few misguided ones we should all ignore anyway). What people go tsk, tsk, tsk at is working really hard and giving up responsibilities at home, not being with you family, even forgoing the biological imperative to reproduce (or delaying it -- usually not a good idea, again biologically speaking), giving up your mental/physical health for a dream that is realistically very remote, etc.
The other big thing is, alas, it is a lottery. We guide our nephews toward a different direction that has a greater promise of economic security: be a neurosurgeon (and well, even if they don't become neurosurgeons, we hope they'll become at least primary care doctors :)). I mean, What is YC's acceptance rate at this point? Like 10%? And I bet the people applying are more able than other hopeful entrepreneurs who don't happen to apply to YC, so the success rate for the average entrepreneur is pretty bad.