Why is that some girls just aren't 'girlfriend material'?

I'm not saying this doesn't work both ways. Some girls will mess around with a guy they're not all that interested in. But I had a friend with benefits who told me he didn't want anything serious, and then got a girlfriend.

So I was wondering, why are some girls okay to talk to and even sleep with but not what you're looking for in a girlfriend? What's missing?

I'm asking this because of my personal circumstances but I'm looking for more general answers too.
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* The title is supposed to read Why is it that some girls just aren't girlfriend material?
Why is that some girls just aren't 'girlfriend material'?
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