It's cause women out here be having mad dudes on deck in case it don't work out with the dude they really like. They got plan B, C, D, F etc guy plus dudes in the friend-zone that gives them male attention and will do anything for them at their beck and call.
Thing is men can do this too but you gotta have superb game, whereas even ugly chicks can have several dudes on deck just by using their sexuality and they don't even have to be sexing any of them.
Since us men approach women, many of us may start dating or talking to one chick and curve every other chick that shows us interest cause we really like one particular chick. And when shyt don't get poppin' were left alone going "wtf?". Whereas for women whether they are seeing someone, dating in a relationship, if they look decent they gon always have dudes crushing on them, chasing them etc.
It all really comes down to hurt people not wanting to get hurt again. They were played so they continue the destructive cycle by playing others. They feign interest enough to the point where all these dudes they got in the back burner think they got a chance without ever having to kiss or have sex with them. By having multiple backup dudes, they don't necessarily have to "face reality" of being alone when it doesn't work out with a particular dude.
These chicks don't like the feeling of being heartbroken, but this ultimately causes their relationships to fail. It causes their relationships to fail because in-between relationships you are supposed to learn from those failed relationships and grow as an individual. You are also supposed to heal from whatever pain, abuse and/or baggage you received from that relationship. And even if their relationship doesn't ultimately fail they are many times unhappy. It's called monkey branching. They don't let go of one branch (guy) until they get a firm grip on the next one.
You gotta also realize that a lot of women are male dependent no matter how much they tell you that's not true. How many chicks out here's lives revolve around their man? Especially if he's really ambitious, successful, is making moves on a consistent basis etc. She's gonna be all about supporting him and his goals. Women like to feel needed by a man, so if the relationship ends that sense of being needed vanishes.
There's also a lot of other things that come into play such as social media and women being bombarded with attention on a daily basis. They don't even gotta be attractive, if they post thirsty pictures or do sexual shyt you gon have simps coming out the wood works.
I already wrote too much, you can write a got damn book on this topic alone.