Why do girls like to mention their "ex"?

It feels like every time I like a girl and start getting to know her she brings up her ex. They don't bring it up as the center of the conversation but just seem to use it in a subtle way like. Oh "you're a football fan? my ex boyfriends family loved football so they kind of got me into it." (something a girl I'm interested and talking to recently said to me)

I really hate it when girls do that, does anyone else notice this? Its like they don't bring it up until you're just starting to get to know each other and there might be that mutual interest here.

To me its a sign of not being able to let go, so it makes me feel like I don't have a chance with the girl or that I'd never be able to "replace" her ex.

Is it a negative or positive thing when girls do this? Or is it just casual conversation with no further meaning?
Why do girls like to mention their "ex"?
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