Writing a properly good song is really difficult. Seems like it should be easy when you see one written down – but you’d be lucky to write more than a handful of good songs in a lifetime even if thats you’re only job.
I think it depends to be honest. A lot of Motown and Stax songs were churned out like a musical production line. Kung Fu Fighting was a B side that took five minutes to write.
Also look at the Brill Building, a songwriting factory, Goffin and King worked there, and the list of songs Carole King wrote for others before she ever recorded any herself is amazing, then there was Baccarat and David, Holland, Dozier Holland…
For some people, writing songs is fairly easy, writing a really successful song is more difficult, and relies almost as much on luck, as anything else, it’s tricky to judge what the public will really go for. Plus a song can work for one artist and not for another, just because a particular arrangement of a song might be more ear-catching. It’s where the term ‘the old grey whistle test’ comes from.