(a) That is what we are told and
(b) That is what is reinforced by those who have been successful
How many stories have you heard that were basically, "Yeah, I started this company part-time, never worked on it more than a few hours a day because I had this other job, and it was bought for 20 million bucks!" Personally, I've heard none.
Now, I'm not going to sit here and tell you that I still work 20 hours a day, because I don't. Last summer I was close, though. I stopped sacrificing my health at the beginning of this year when I began going to the gym every day again and stopped smoking a pack a day. Now that spring is upon us, I'm even beginning to remember a time when "woman" was a word in my vernacular.
The reason you see these types of articles and the reason you will continue to is because for those of us who have no other jobs, our startups are our lives. I could work for Big Co. and go home at the end of the day not at all worrying about whether or not they will be there tomorrow. The company would pay me X to perform Y and that would be exactly what I'd perform (if I wanted a promotion, maybe Y+1).
At TicketStumbler, I am not merely a cog. If I don't write code for that new feature or fix that bug or setup that new server, nobody else is going to. This means that not only do I constantly have a lot to do, but it all has pretty much the same priority: done yesterday. And if I decide to take a few days off instead of doing one of those things? I get the pleasure of feeling guilty about it the entire time.
So it basically boils down to: at an early-stage startup you have the pleasure of controlling your destiny and being your own boss; for those of us who cannot stomach traditional employment, we feel as if we actually have a soul. The downside? Well, you live in a constant and unrelenting state of stress and obligation. The effects of that state differ from person to person, but alienations/estrangement from family and friends and unwillingness to prioritize anything other than the company are common ones.