Every Pop, Rap, Hip-Hop, and R&B song has music in the background and an overwhelming majority of the music, beat, melody, whatever of those songs are not produced by the person credited with ownership of the songs. One Direction, for example, makes absolutely none of their music. They get handed a rhythm, a tune, essentially a song without lyrics, and they just sing to it. Rappers like Jay-Z and Kanye West become hugely popular because of the catchiness of their songs. Where does (arguably) half of that catchiness come from? The music. But, will you ever see "Jay-Z and Joe Moosicmakre - 99 Problems" or "One Direction - What Makes You Beautiful (Featuring Sally Songwriter)" on the Billboard Charts? Nope.
Why do you think this is? Why are musicians so reluctant to give credit where credit is due when it comes to the people who produce their music? The process of producing the actual music for a song is arguably much more difficult than essentially writing rhyming poetry that goes to the beat thereof.
One example of a guy who does it right is Macklemore. Ryan Lewis is the person who produces all of his music, which is why you see "Macklemore & Ryan Lewis - Thrift Shop" every time you go to YouTube for your fix of the song. That Soprano Saxophone hook in "[url=http://youtu.be/QK8mJJJvaes?t=19s]Thrift Shop[/url]"? The Trumpets in "[url=http://youtu.be/2zNSgSzhBfM?t=4m3s]Can't Hold Us[/url]"? The Piano in "[url=http://youtu.be/hlVBg7_08n0?t=20s]Same Love[/url]"? All Ryan Lewis and all duly credited. But, people aren't ranting and raving about Ryan Lewis, they're doing it all about Macklemore, and they both know that. That still doesn't stop Macklemore from continually giving credit to Ryan for all the hard work he puts into all of their songs. I get that they're clearly close friends and the situation is a little different, but what's wrong with crediting one's music producers?
TL;DR: Why do you think most Pop, Rap, Hip-Hop, and R&B artists don't credit the people who produce their music?
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It's not often you hear an artist openly credit their producers, but I'm sure they thank everyone who worked and supported their projects specifically at award ceremonies.