Most Helpful Girl
Because there are many more guys than girls on those sites, so if you're a pretty girl you're being inundated with messages and date requests from men. It tends to make you pickier because you're making direct comparisons between people. Plus it's even easier to fall into that because people are reduced to their pictures and whatever facts and info they choose to share about themselves.
In contrast to meeting someone in a bar or somewhere else individually, where the guy is not in direct competition with so many other guys.
For instance, online you might see that a guy's a smoker and eliminate him outright, whereas maybe if you met him in person, got to know him and liked him, it could've been something you'd overlook.