Personally theory.....

Many startups fail because people try to work 80+hrs per week. Biologically more than about 25-30 hours of work is usually a waste of time. You can occasionally pull a long week but then you need to rest and recover to get back to full productivity. If you push beyond it often, you'll make a shit ton of stupid mistakes that completely nullify all your efforts.

If you've ever been around someone "working" on hour 70+ during a week you'll know what I mean. A five minute tasks takes them an hour and they generally fuck it up.