Well, not everyone believes that. The question was in response to why someone would dogmatically cling to theories that aren't supported by the evidence.
I'm not interested is psychoanalyzing people. Einstein resisted quantum theory to his death. Inability to change is not necessarily indicative of stupidity or dishonesty.
However, Duesberg has destroyed his career by accusing the entire worldwide medical community of being dishonest. If he were correct, the most effective thing for him to do would be continue his research and publish results without calling everyone who brings evidence contradicting his theories a liar.
Back around 1981-2000 he had a good part of the gay community on his side. He has lost that support, as he has lost the support of people like Magic Johnson.