When you are constantly talking to a lot of people every day it just happens naturally. Most people date through their friendship circle or through work.
Think about it this way, most people spend hours every day talking to people. When they do that, think about all the personal relationships they build up. Now think about all the people that trust them because they built those personal relationships. Once that trust is built, it's pretty darn easy for two horny single people to hook up with eachother when they are in the same room.
I used to wonder this too, until I started going out more and observing people. I also observed my friends when I was in a social circle and got a good idea of how things work. At first I wondered how they got so many girls, and then over time I actually saw how many people they talk to on a daily basis. When you know so many people, all it takes is one person to make a move and then things can escalate from there.
In simple terms, most people have pretty wide social circles meaning they have many options.