Can’t speak for everyone, of course, but it’s my general impression many authors don’t have great faith or confidence in their own work. They are mostly a shy bunch, the ones I’ve met anyway, backwards in coming forwards.
There are exceptions of course. Norman Mailer was hardly shy for example. He seems to have been aggressive in the face of pretty much everything. He once got in a near fatal fight in the street with three sailors because they said his miniature poodle was ‘gay’. I suspect his confidence was boosted by the huge, unexpected and breath-taking success of his first book The Naked and the Dead. After its publication he was feted across the USA like a movie star. It was Martin Amis who pointed out that someone in the UK whose book had a similar level of success would buy a set of filing cabinets and consider giving up their job as a school teacher.