Asked in New Testament
Why do people not believe in Jesus when there have been witnesses?
It depends on what we mean by 'witnesses'. People do not believe in Sherlock Holmes, although the books tell of many witnesses to his exploits. Most people do not believe in the events of the Da Vinci Code, although it is written very plausibly, describing a real historical background and real people who were peripheral to the story itself. With these books, the lack of belief is justified because witnesses have to be independent and credible.
So it is with the gospels. The disciples described in the gospels are not independent and have not attested to the existence of Jesus or to his miracles, outside the gospels in which they are characters. Paul is both independent and credible, but he never showed any interest in visiting any of the holy sites associated with Jesus and never seems to regard Jesus as a man who lived on earth in the recent past. Whether he genuinely believed that Jesus had once lived on earth or that Jesus was a spiritual Messiah, this belief could just as easily be ill-founded if it was based on faith alone.
Apart from Paul, there is no contemporary or near-contemporary of Jesus who could be brought forward as a witness. Even Philo of Alexandria, who lived around the same time and wrote of all the Jewish religious movements that he was aware of, never mentioned Jesus.
This is why there are people who do not believe in Jesus.