Do you mean, some people don’t HAVE beta readers? How can that be?
I can’t imagine how maddening it would be to actually publish a book without first hearing from several people whether or not it works. My sister is usually the first one to read my work and as much as I appreciate her feedback, she’s my sister.
I would never have the courage to hit publish without first hearing from other folks, like my writing group, who are not afraid to be brutally honest and have fun tearing apart my paper thin plot maneuvers, my repetitive sentence structure and my abundant spelling and grammar errors. And while they are invaluable—I’m mean really and truly the greatest group ever—they usually read the work in thirty page chunks. It’s difficult to judge if a plot holds together when you don’t read the whole way through.