Songwriting is a craft. People who are gifted with artistic talent but fail to learn and master the skill of songwriting produce unpublished crap.
Talented, gifted songwriters have to work their asses off to acquire the skills necessary to elevate their craft to a professional level, then they have to work their asses off crafting every single song they write. Every once in a blue moon, a songwriter who has spent all that time learning the craft and creating songs will write a song in a day or two, but it's quite rare.
How many millions of 'students' of songwriting/writing/movie making are still waiting tables? Every waiter in NYC or LA is an actor/writer just waiting.
Art is a gift. Skills can be honed and refined. Talent is inate.
As you say, it is very difficult. Lennon and McCartney were trying to be Goffin and King when they started. That they were mere kids and were able to write so many songs that remain classics fifty years later is simply stunning.