So many budding authors start with fan fiction. The majority had never contemplated writing stories beforehand.
The entry point is extremely easy, especially on the internet. There are multiple channels for publishing fan-made stories on-line.
Wherever the fandom meets, in forums, on art sites, or in bespoke fan fiction websites, it's as simple as clicking a couple of buttons and pasting words into a box.
Communities build up around them, so there's plenty of peer encouragement too. If everyone else is doing it, then you might as well give it a shot.
Announcing that this is your first ever story will have proud club members clucking around you, like a mother with baby's first painting. And if you do attract a troll, then the rest of the community will take them down. But only for the first story, or until your noob status wears off.
Writing for the fandom allows ample opportunity for sampling and experimenting with different literary genres.
There's only so far you can go with the original story before a sense of sameness builds up. Therefore one way to stand out is to keep the plot and characters, but rewrite it within a different tradition of story-telling, or in the style of *insert famous author*.
The community may help here. Fans and fandom clubs are always holding competitions - write an Easter story; or make it sound like a Disney film! (Always interesting, when your base material comes from one of the darkest manga stories ever written.)
Fan fiction has an unfair reputation for being too easy. The characters and universe are handed to you by the original author. Therefore, observers conclude, the rest must be 'color by numbers' for writers.
As I've already outlined above, the reality is that it's harder. But those dipping their toe into the writing pool for the first time don't know this. They've heard the hype; and they have no experience of going it alone.
I should clarify that fan fiction can be extremely easy, but only if corners are cut. If the fanfic author announces from the outset that this is all a bit of fun, experimenting with an alternative universe, then all bets are off.
It can also be a simple piece of formulaic story-telling, if the writer merely retells canon events in their own words. But their readers have been here before (with 90% of other fan fiction writers). It's not only been done to death (particularly the most popular scenes), but an original already exists, immutably better by default.
By taking the easy route, these writers won't build a substantial readership. The curious will come, then drift away.
Nevertheless, it is writing. It's doing so in the deep end, where the sharks are lurking behind the pretty shoals. Those who make it in fan fiction will eventually turn to original writing. They will find it much easier.
They will also now have experience in word-crafting and a vast archive of reader feedback. This author knows what is a crowd-pleaser; and what is universally disdained. This is invaluable knowledge, which can then be transferred to other, potentially lucrative, avenues of writing.