The reason why this happens is songs are written by committee these days.
Major pop stars hold writing camps when they start a new project.
There was video with a guy explaining the process but I cant find it.
I found this article instead, which states the same as that video.
BBC Arts - BBC Arts - The pop-up hit factory: How a songwriter bootcamp helped Rihanna fight back
A-list producers and topliners were summoned to Los Angeles for two weeks and installed in studios around the city, their hotel and food all paid for. It was the kind of immensely expensive undertaking that only a major label can afford to arrange.
Nowhere are the production efficiencies of the track-and-hook method of writing better realised than in writer camp. A camp is like a pop-up hit factory. Labels and superstar artists convene them, and they generally last three or four days.
The usual format is to invite dozens or more track makers and topliners, who are mixed and matched in different combinations through the course of the camp, until every possible combination has been tried. Typically, a producer-topliner pair spends the morning working on a song, which they are supposed to finish by the lunch break. In the afternoon new pairs are formed by the camp counselors, and another song is written by dinner.
If the artist happens to be present, the artist circulates among the different sessions, throwing out concepts, checking on the works in progress, picking up musical pollen in one session and shedding it on others.
At the end of each morning and afternoon session, the campers come together and listen to one another’s songs. The peer pressure is such that virtually every session produces a song, which means twelve or more songs a day, or sixty a week, depending on the size of the camp.
The Rihanna writer camp was an elite affair. Ne-Yo was there, as was [Scandinavian production duo] Stargate, and Ester Dean. Jay Brown played counselor to the campers, under the aegis of Def Jam management.
The reason the labels do this is because they are trying guarantee hits by treating music like a GMO food product.
Take his drums, take his samples, take her verse, take her hook, take his keyboard part, take his bassline and put it all together.
By the time it's done, you have 15 writers splitting small percentages of the total songwriting publishing.
The artist who the song is for will get writing and production credit and a big slice of the publishing, whether they contributed or not.
Most times, they don't contribute anything,