Why do people lie in court?

Anyone who is involved in the legal system knows that court cases are filled with people who have vastly different recollections about events. Some of that might simply arise from mistaken impressions, but it might also be because of people lying. However, what is clear is that people are rarely convicted of perjury and police are virtually never convicted, even though there is substantial evidence of them lying. A law professor friend of the CPRC, who has practiced criminal law and taught it, summarized for us his class lectures on this issue for us.