Why is it that girls often start nice, and then ultimately they end up ignoring you?

It happened to me many times, and I am always left in disbelieve. They don't even act on a friendly bases how could they be so cold? What kind of animals are they? I don't act like that towards people I like. And I mean its just a return message we are talking about here to show some most basic respect. But not even that, they ignore the message. And as I said in the question they start nice that's the odd part.

Updates
10 mo
Guys, apparently it was a false alarm! I just can't believe it. She did answer me today in the casual friendly tone that I was expecting. I just don't understand why she would reply to a basic message 2 days later after I almost unfollowed her!

P. S we never dated or anything so it should not be a complicated interaction. And we live in different countries. We are basically on each others' friend list on Insta. We only met by coincidence while on vacation.
Why is it that girls often start nice, and then ultimately they end up ignoring you?
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