I think some people want to share their happiness by writing about their earnings. It may land on a reader like bragging. It could also be a click bait.
I am writing in Medium because I
1. was fascinated by the top quality articles from some authors
2. I was inspired to try self publishing
3. Then Publish for other Publishers
4. Then I tried MPP. Made my first 1.86 cents. I feel more happier than when I was a Manager making way more money.....haha
5. Like me other people may have their own story. As Medium gives you to writer and self publish each wants to express I guess..