There is absolutely nothing to keep a judge from saying what he thinks, and there is absolutely nothing to prevent a senator from casting a confirmation vote based on what a judge says he thinks.

So, in order to keep from giving senators anything upon which to base a vote, apart from the carefully groomed "good person" image, it's safer for a nominee to project a tabula rasa personality. Of course, We The People get a pig in a poke that way, but whatever.

Until the last half of the 20th century, men were appointed to the Supreme Court more or less on reputation alone, especially if they had political connections, with little investigation. It was not even known that Hugo Black, who turned out to be a very good Supreme Court justice, had been a member of the Ku Klux Klan as a younger man until after he was appointed and confirmed.