Because they would rather embrace their religion and legalism that they could see than put faith in God whom they could not see. Today there are believers who do not set the Word of God as a priority in their lives. They are no different from the Jews in this passage. Man can say he believes the Bible exists. Yet that is observation not faith.
Man can say the Bible is accurate in what it presents.
Yet that is good scholarship, not faith. Man can say the Bible contains good principles for living. Yet that is only common sense and not faith. Man can say the Bible is from God and it is true in its entirety, revealing God's Word for man. And yet that is not believing the Word of God.