If you listen to the Beatles' own accounts, they didn't appear to pay much attention to authorship until near the end of the group. They got together, kicked around ideas, made suggestions in a more or less organic way with little attention to source or credit. It was a workman-like "git-er-done" approach. The reason they grouped "Lennon/McCartney" credit together early was in part to make collaboration smoother.

In the latter years of the group, they became more possessive and isolated, for whatever reason, and collaboration was less organic. It illustrates that money and fame DON'T make one satisfied. Egos and personal demons lived on and perhaps got magnified.