Because nobody knows what it's like to write and finish a book.
Are we writers, then? When people ask us what we do, are we entitled to label ourselves as writers? Or does a paycheck constitute the label?
Even now, with a book deal, I find it difficult to say I'm a writer. I get shy when people ask me about my work because I'm not quite sure they understand or rake the writing process seriously. But then again, maybe I need to lead by example. I need to show people how serious I take my job in order for outsiders to do the same.
And if they don't? Who cares? In living my dream, and it's very likely they are not. So keep writing writers. Call yourselves whatever you want. But no matter what, if you write you are entitled to the label: writer.