It is amazing any book has been written and completed considering the distractions of daily life. How does anyone manage to complete a book?

 It is amazing any book has been written and completed considering the distractions of daily life.
How does anyone manage to complete a book? 

Writing is a solo practice for me, anyway.

I have never experienced writing anything in the company of others. I do not require assistance.

How have authors throughout the ages managed to complete books while dealing with
outside distractions from people and things that want their attention?