> Is it not possible to lead a startup and work normal hours (≤8) and spend time with your kids?

Absolutely. The problem is that if you go head-to-head with people that will go all out there is a chance that they'll be able to outgun you with a similar sized team.

In practice there are ways to offset for that depending on the situation, for instance, you could have more experience, better quality people you're working with, an established track record and/or a better launchpad.

So it's not just a one-dimensional (time put in) problem.