Well. First of all, I think that it is much easier to continue a story than it is to start and finish it correctly. If the author knows the characters well, the story might be endless, finishing only with their death, and there is no way to stop the characters that want to tell their stories unless you kill them, and even after that, there may still be their children left... I'm not even joking. My stories always ended with the death of all the characters because they just won't stop living and the things continue happening to them... so, when I was writing my own story in my school years, I had an outlined plot for 5 generations of characters and the previous characters were almost all dead, and of course I didn't write it all, I wasn't able to, I've only written one book. Before that, I only wrote short tales about animals and all of them died at the end because of the same reason.
Also, I don't know any of the famous writers so I can't ask them about it... but among my friends, there are different ways of writing stories. I have a friend who is excellent at plotting. He writes an entire plot and then he just follows it. It seems strange to me, because I have to listen to what the characters have to say about themselves, and so I can't have any plans. I can have only a vague plan because they never tell me things in exact order. (And that's why I rarely finish things). I tried to write according to a plan and I kind of succeeded and people said that it was good, but to me, it was dead, because I didn't talk to these characters, they told me nothing about themselves, the story was over before they had a chance to talk to me, and I never got to know them. And yeah, I'm one of the people who don't write a story if they already know the ending.
I think that all ways of writing are good as long as the author is content with the work, even though it is clear which way I prefer.