Why do friends ask if you want to hangout, then bail on you?

So my friend texted me asking if I wanted to hangout sometime and I said “sure, yeah! Just let me know whenever” and she never responded. Two days later, I texted her, “do you still want to hangout?” And I still haven’t heard from her. But then, when she sees me on campus, she’ll wave and say “hi” to me. It’s been a week since she’s asked me to hangout, and she still hasn’t responded, but she posted a hiking picture on her Instagram. Why do friends do this?
Why do friends ask if you want to hangout, then bail on you?
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