Credits on a current single frequently list half a dozen writers or more, which reads as a decline in individual authorship. The list is better understood as a record of who owns the composition.
A credit is an ownership share
Songwriting credits allocate the publishing rights in a composition, and those rights generate income whenever the song is performed, streamed or licensed.
Anyone whose contribution is recognised as part of the composition receives a percentage, and the percentages must sum to the whole.
The list is therefore a legal document rather than a description of who sat with a notebook, which is why it can include people who were not present.
Samples and interpolations import old writers
Using a portion of an existing recording requires clearing both the recording and the underlying composition.
Clearing the composition typically means assigning the original writers a share of the new song, so every writer of the sampled work joins the credits.
Interpolation, where a melody is replayed rather than sampled, has the same effect on the composition side, which is why a song borrowing a familiar line acquires several historical names.
Production became part of the composition
In much current music the distinction between arrangement and composition has collapsed, since the beat, the chord voicings and the sound design are the song.
Producers who supply those elements have a reasonable claim to a compositional share, and industry practice increasingly grants one.
Where several producers contribute at different stages, each addition brings another credit, and none of them wrote a lyric.
Sessions are built to be collaborative
Writing camps assemble numerous writers and producers to work in parallel over days, generating material for a specific artist or project.
Ideas move between rooms, and a topline written in one session may end up on a track built in another.
Reconstructing who contributed what afterwards is genuinely difficult, so splits are negotiated as a settlement rather than determined as a fact.
What the arithmetic does to earnings
A share of a composition is a share of a fixed quantity, so more names means a smaller portion each.
For a widely streamed song the total can still be substantial, but for a modest release divided many ways the individual amounts become small.
This is why writers negotiate splits carefully before a song is released, since renegotiating after success is far harder than agreeing terms in the room.