Why don't girls add guys that they're interested in on FB?

There are quite a few girls in my circle of friends/mutual friends (in real life) that seem to be interested but never send me a request, If I'm the one to add them then its all on me to start chatting them up and my nerves always get the best of me, Maybe if they added me first it would make things a lot easier
Why don't girls add guys that they're interested in on FB?
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