Well, for me the primary reason I look for other friends -- and that's just-friends at this point -- is because my poly GF is unable to spend as much time with me as my emotional well-being requires.
This is new on several levels for me: coming out of a 30-year mono marriage and starting to date again was quite a change but to end up with a person who's in a poly marriage should have been a double shock to my system. Oddly, it wasn't -- the relationship feels quite normal. But there's no competitive urge, or any desire to "catch up" to the poly couple I'm involved with.
I do get lonely, though, and the depth of my relationship with L has opened up my mind to what is possible. So that intrigues me: is there someone else with whom I could connect at a similar level? And that curiosity and desire for another deep friendship is what keeps me looking.
My personal take on the situation: suddenly people are very interesting, and women in particular because I open up faster and deeper to women than men. You don't have to be looking for physical intimacy; the search for someone you can really share feelings with is enough reason to look. IMHO.
But mileage varies so much that any blanket statement is going to be wrong for some people.