The number of hours one can work a week productively will vary based on individual and lifestyle.

But to me, currently working 60 hours in my startup (sleeping well, eating well and exercising, without children and with a good social life). 40 hours does not seem like much. Am I missing something?

If I worked only 9-5, I would pick a side project, as I would feel like I wasn't challenged though.

I understand having a family would change this. I can see how if I had 3 kids, a 30 hour week would be a blessing.

But we can't assume the same optimal number of hours for everyone. 40 hours is nothing for a single person with few responsibilities.