Why do girls only show interest when your already in a relationship?

This has happened twice already (but multiple time in my past two relationships) but when I'm single it's like women just want to be friends or just turn me down but when I do get into a relationship some of those same girls will come back out and start showing interest why do they do this it's kinda frustrating can anyone share they're opinion on this
Why do girls only show interest when your already in a relationship?
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