It's not enjoyable for guys if meeting is failed, especially on public, in front of other people.
And the girls don't want to impose relations to the guy; everyone wants the communication and acquaintance to start without any efforts, freely.
Why should you continue communication with the person that you're not interested in?
In Chatroulette you just press the "next" button and the chat will switch you to a new interlocutor.
For lots of people it is quite difficult to talk to a stranger in the street and the more to get acquainted.