Engagement is not nearly the same. There are 1.5 billion WhatsApp users, but how many would actually convert. I'd find it hard to see numbers higher than the low single digits.
It's not too difficult to imagine all sorts of fun bonus features in a chat app.
That seems to indicate that 22% of Line's revenue was from stickers, which was lower than the highest section: advertising. Those are 2017 numbers, and I'd suspect that advertising makes up an even bigger section now.
There are people who spend 10 hours a day playing Fortnite, and are perfectly willing to spend gobs of money to make that experience better. There aren’t the same sort of “whales” in social media.
Video games are not fundamentally different here, I think you might be looking through the wrong end of the telescope. People who use a thing paying money for more features of that thing.