Because your VCs will go crazy if they found out you were only working 8 hours a day. I've interviewed with both TechStars and YC startups - and they've told me that 55-60 hour work weeks are the norm. Damned if you do, damned if you don't.
One huge winner could randomly happen in an otherwise low-quality program and that incubator would appear higher on rankings forever after that unjustly since the result is very much not reproducible. In other words measuring by average could mask getting lucky.
One would think that. The article states that 45% of programs have not had any graduate that raised venture funding. That boggles my mind. But this data helps explain the uneasy feeling I get when I see niche accelerators following a cargo cult mentality.