It happens but it is rare.
It is hard to find someone you fall in love with and who falls in love with you.
It is harder when you are looking for someone to fall in love with both of you, at the same time, and who you'll both fall in love with, at the same time.
I've heard of more Vees turning into triads than triads starting that way, I believe. That is, one partner had a girlfriend, who eventually became close to the other partner as well.
You can keep looking, but it's going to be intimidating for a bi girl to be faced with two people at once and have to deal with two relationships at once. That's why it's more likely she will, or already has, find a boyfriend first. As a result, most of the bi females wanting a triad are already paired with a boyfriend. Since the boyfriend is usually not fine with his girlfriend dating other males, two couples cannot become two triads (the two females being with each other and both with both males), which otherwise would probably have higher odds of finding someone.