That difference is the astonishing bit.
Each Beatle album getting the 'best' of a group of talented writers was the magic sauce, imo.
But even then, the pace was still... insane. Tour in the 1965 summer time, come back to UK, start writing an album in October, work for a month, and put out Rubber Soul, nearly all of which was written and recorded in that single month. That is possibly the most creative/productive single month in their career.
My understanding that back in the day this neck breaking pace was rather common, often because of pressure from the record labels and contracts that bands were forced to sign with them.
Absolute majority of known bands out there are "one song marvels" and those that are not are rare, well know worldwide superstars.
You may need to clarify your criteria there a bit more for "good song". Just chart position? Memorability? Lyrics? Catchiness?
The only composers in history that I'd compare that track record to would probably be Mozart and J.S.Bach. Admittedly both Mozart and Bach were individually creating those pieces, but the Beatles were a very rare meeting of musical minds. I even rate some of Ringo's tracks as being good compared to the average song writing of the last few centuries.
But, as I said, I'm far from unbiased here!
There's also the Paul showing his music to John, and John telling him it was crap and to do better. And vice versa. After the breakup, who is going to have the nerve to tell Paul that his latest creation is crap?