I have a FWB question........?

Ok..i was just wondering when two people are just friends with benefits and there "relationship" is just physical, how in the world do feelings develop by one or both people when a certain amount of time pes after agreeing to do the friends with benefits thing? I don't understand it because, fwb is just physical with nothing else, its not like both people are ever going to be together and its not like they really care about each other except to get their "benefits" of being each others "friend". And how in the hell did this fwb thing ever get started!?