It was the other way around, Fanfic with Self Insert protagonist is usually hard to find. And finding the ones that didn't suck is even harder.
Fanfic is alterations or fanmade story of official story by some fan that want to change some thing inside the story.
Like for example, when someone write Harry Potter fanfic, you can expect some sorry like :
"Harry Potter is having gay love with Snape and Malfoy, the whole character have painted hair and tatto like ghettos, LGBT rainbow flag is flying everywhere inside castle, painting in the wall is cursing the name JK Rowling, Teachers are doing Fortnite dance, Professor Werewolf is marrying Prince of Azkaban, Hermonie is going acceleration course and will graduate next month, Ron and friends is forgotten (cause writer thinks Ron is stupid even tho he's just a kid), and Hogwarts headmaster is not Dumbledore (it's Voldemort btw)."
That's fanfiction, a fan made story that based on already existing story.
Sometimes when they want to be the "modern person" into the story and they getting along with Main character and friends as buddy while being powerful and knows everything guy. It's Self Insert, and usually suck because they just want to be strong inside their own story.
It's only when the Isekai Boom started, with SAO and others, that many people started writing High quality story of people getting sent into the fantasy world
But the trend of "reincarnated into story" only boomed after "My Next Life as a Villainess: All Routes Lead to Doom!"
This is the core of stories about people getting transported into novels, Especially as villain.
Its the reason why all self insert fanfiction suddenly became far more popular to the point of overwhelming normal non self insert fanfiction.
