Mainly because most men don't want to commit to a long term relationship. Most people don't want to upset others. If men told the truth and was honest it is likely people would feel a bit degraded in someway.

Most men would love to be honest but it is pretty unlikely that a woman would want to be with a guy who says he is interested in her mostly for sex. It is unfortunate but it's the nature of male and female relationships that they are often looking for different things. Although ulitmately they both want healthy offspring.

Its surely strange but people are basically selfish, they feel that they are too smart for others..
I feel this makes them feel safer..