Ask HN: Startups and Past Contributions

In many startup-related articles and HN posts, I've read that people tend to overvalue past contributions, and undervalue future contributions. Eg: http://cdixon.org/2009/08/23/dividing-equity-between-founders/

Why do many people not put a lot of value on past contributions, such as forming the idea and developing the business plan, when deciding how to split equity between co-founders?