in my experience it's actually pretty common for newbie fanfiction writers to gravitate towards coming up with OCs rather than writing about the canon characters (hence the infamous Mary Sue); this suggests that many people find that 'easier' than attempting to write about pre-existing characters.
I would say that recreating the characterisation, mood and general vibes of an existing work of fiction is in fact a skill in and of itself that's acquired via practice. A common compliment delivered to fanfiction writers is how well they have captured the spirit of the original work. Inexperienced fanfiction writers fail to do this.
When creating an original character you can, ultimately, say anything about them and now it's canon bcos you created them and you said so. conversely, well, if you are covering a well known song and start playing the wrong notes people will notice immediately.
In addition: a lot of fanfiction, IMO, is in actuality a form of metafictional literary analysis. a story where two characters sit down and talk about their feelings about a canonical event? literary analysis, which is of course a skill. Sometimes fanfiction is bad bcos the writer is not good at interpreting texts.
finally I have made this point before but you never really see anyone suggesting that fictional works based on existing public domain texts are on 'easy mode'. is it 'easier' to write something based on a copyrighted work than a public domain work? Of course it isn't.
