Why would they keep me in their phones?

I've know dated 3 girls, and we've long stopped dating.

1. Dated her for 8 months, and she always texts me when something goes wrong, or she needs help with something. And we didn't even end on good terms. We haven't dated in 3 years, but she still keeps texting me, Even when I won't help her.

2. A friend through my senior year in highschool, and I hadn't seen or talked to her in 4 years, and one day I stumbled across her on a dating site. We only went on one date, but she kept my number all the years, but never texted me.

3. A girl I worked with for a year, we didn't talk until recently, when she was texting me wanting to hang out and go out. But now she's not talking to me again.

Why would they keep my number and just not talk to me?
Why would they keep me in their phones?
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