It's show business, common industry practice there's a lot of threads on here on the subject and some older posters with industry ties have exposed some secrets. The first rap hit was ghostwritten.
The other day Honorable SKJ spoke on Skillz and a NY mixtape rapper who he did not name writing for Ma$e on Harlem World I'm guessing Cardan, not to mention verses lifted from Cam'ron from the Children of the Corn days both guys recycled verses. Ma$e wrote B.I.G.'s verse on 112's Only You and supposedly wrote his verse on Mo Money, Mo Problems too.
Art Barr has spoken on MC Juice writing Summertime and not Rakim for Will Smith. He's spoken on Gillie and Mikkey Halstead ghostwriting for Wayne (he sued Cash Money). Nicki also has writing credit on Lollipop and Drake on I'm Single.
There was a producer out there who claimed to have wrote verses on Jay's B*tches and Sisters. Also, the rumor that Lupe wrote on The Black Album exposed by one of Lupe's mans. Ma$e has writing credit on I Just Wanna Love U.
There was a thread on here talking about Stat Quo, Yung Berg, Roscoe Dash, Skeme, Kid Ink, Ty $ being major industry ghostwriters.