It's all about them being selfish by keeping everything that they already had. They want to reject you, but they don't want to be rejected. The "friendship" ENDS when you become sexually attracted to someone. Either you evolve or dissolve. Think about this. What do friends do? Hang out, talk , there for you when you need them, go places, try to hook you up with someone, constantly. Those are examples but you get the picture. She wants to keep you around for the time being, not considering the feelings you have for her is going to hurt when you see her with someone else down the line. Some guys think that if she is still in his life there is a way of getting her back someday, and that's just wrong.
By being her "friend" what does she get and you get? Lets compare. She still has you around so there is no feeling of loss, knowing she can have you anytime she wants she can set the pace on where and when she wants to see you or communicate with you, she has the freedom to be with other guys and since you are her "friend" you have no say in the matter, she already has lost the attraction and feelings for you so by you being there it helps her swim to shore while you should let her drown in feelings. When she finds another guy (sooner than you think) she will tell you excuses like "it just happened" while all around insulting your intelligence.
Now what do you get? Watch her move on and flirt with other guys while you are still in the "picture", you haven't moved on from her and you have false hope of getting back together, the awkwardness of being alone with her and when you make a move she will tell you "wait we are just friends", hanging out with friends and her at the same time will get ridiculous especially when your other friends take her side and hit on her, when she finally finds another guy you will be forgotten about instantly because the new guy gives her what she wants and that includes the attention that you were giving her.
These were just examples and there are much more. So as you can imagine it would be great for your ex and not for you, especially if you haven't moved on and if you are the jealous type. Moving on and cutting an ex out of your life totally is the best way to go. Is it possible to be "friends" after being dumped? Picture this. If you are no longer attracted to her, if you can watch her with other guys and imagine her banging them and not lose your mind, be an emotional tampon for when she needs your "help" or someone to talk to as one of the girls, then yes you can be her friend until she milks you dry from attention and comfort until she dumps your friendship and you will start all over again with trying to let her go and move on. I say move on, say NO to "friendship" because you have real friends already, and go find another girl that will appreciate your worth as a potential mate not as one of the girls.