It's because most women get close to people they work with. They start talking about their marriage/relationship problems at work, interested men start lending an ear and then become the person the woman confides in. The woman in turn starts developing an emotional attachment to him and the panties end up coming off.
Let's not give MOST women too much credit. They feel something is missing, start taking to people about it and then wind up developing that emotional bond with them and eventually screw them.
Some women are attracted to the strictly physical aspects of sechs, but most are emotionally driven. Men are physical instead of emotional about sechs for the most part, especially early in a relationship.