Why is it that most couples that post endlessly about their relationship on social media fail so horribly?

1- my sister and her husband. They were both cheating on each other and it ended VERY messy. Yet they were always posting how in love they were.
2- a coworker and his wife. They also posted endlessly about how they were soul mates, and unfortunately both died in a murder suicide last month due to endless cheating.
3- a guy on Facebook posted pictures of he and his girlfriend yet has sent me multiple messages to my inbox telling me how beautiful I am, how he wanted to take me out, etc, all while posting about his girlfriend PUBLICLY! And ofc I blocked him.
++many many more situations like this. WHY are couples like this?
Why is it that most couples that post endlessly about their relationship on social media fail so horribly?
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