Well, some of them do. But many don't. Dan Ariely writes:
We picked apart emails sent between online daters, prepared to dissect the juicy details of first introductions. And we found a general trend supporting the idea that people like to maintain boring equilibrium at all costs: we found a lot of people who may, in actuality, have interesting things to say, but presented themselves as utterly insipid in their written conversations. The dialogue was boring, consisting mainly of questions like, “Where did you go to college?” or “What are your hobbies?” “What is your line of work?” etc.
…What we learned from this little experiment is that when people are free to choose what type of discussions they want to have, they often gravitate toward an equilibrium that is easy to maintain but one that no one really enjoys or benefits from.
First, email is a bad medium for making and negotiating outrageous claims. You can't communicate subtleties of tone and teasing and you can't easily do "repair work" if you offend the audience, even assuming you can notice the offense or keep the dialog going after an offense.
More generally, when I see cautious behavior I ask if some kind of threshold incentive is in place. Imagine a process where both the writing man and the writing woman have inferred they are above the other's minimum standard for a personal meeting, perhaps for having demonstrated looks, money, status, etc. Given that a relatively impressive credential already puts each person in the running, on the written exchange each is simply hoping to "break even" for the time being and avoid dismissal. Draw your own implications as to what attracts people in the on-line dating world.
Addendum: Robin Hanson offers related comment.