Some authors excel combining the roles of writer and marketing guru, but it's my sense that most do better at one or the other. It's probably ideal if you can hire someone to do the marketing for you, but that typically costs more than most writers can afford.

Some authors are really in a bind. They're told they have to market their books aggressively to succeed. But they may lack the skills to do that or not be able to hire someone else. It can lead to a lot of self-blame or victim-blaming ("well, you WOULD have done better if you'd marketed your book more better") when, in many ways, the entire system is broken.