Why do people feel they can behave in a way that can be blackmailed.

Just behave

Yeah, how dare someone do or say anything that some random crazy asshole could use to threaten that person's personal or professional life or even put them in danger of physical harm.

To hell with gay kids growing up in very traditional religious areas in much of the world.

That person who made a racist joke on Discord when they were 13 years old? That should be able to ruin them when they're 30!

Someone confiding to a friend over social media DMs that they're in an abusive relationship with someone violent? Well - she shouldn't be surprised when her partner beats her within an inch of her life when he finds out. If only she did what she was told, right?

And let's not forget the cringiest or most sexual thing you've ever said online - make sure that your every utterance in private would pass scrutiny by your employer's HR department!

Seriously...I don't understand people like you. What a small, listless, and unusually safe world you must live in.

You may as well have asked why can't everyone think and act like you as well as live in your particular region of the world with the same friends, family, romantic, and professional opportunities that you've been provided throughout your life.