There is something about internet anonymity that really allows people to feel comfortable bending the truth or even flat out, blatantly lying. There are also people who feel totally fine fibbing to the friends and family on their timeline. Why? Well, some people do it to make a political or societal point. Or to express that they have some kind of experience with pain or strife. You know the type of situation or person. Rachel Dolezal. People who pretend to have been denied or given bad service because they were white or don't believe in vaccinations or disease-preventing masks.
Other people weave their flourish-y creative tales for attention or pity. They dream up fake cancer diagnoses for sympathy and con friends, family, and strangers in order to rake in the crowdfunding money. Some of them create a sob story so that people will talk to them on sites like Reddit or Twitter.
Then there are the attention-hungry individuals who are looking for clout. They write up detailed sagas to mine Reddit karma, which will in turn allow them to post their lies freely all over the website. Or they want their tweet to go viral. And then there are the wannabe tough guys who regale the internet with fictitious heroics or even more false tales that evoke the classic "I have a girlfriend but she goes to another school" trope.
You'll find that most of these types are represented in this lengthy list of creative storytellers. While we do wish that people would just, you know, not lie on the internet, at least there are places like r/thatHappened where we can laugh at their foolish attempts.