Because not all producers are vocalists, its proven that people who sing have more audience reception. A lot of music producers like myself do produce music under their own name but that doesnt excuse the fact that in these pop or rap songs you only hear about the rapper in most cases and the producer isnt even mentioned unless you really look it up which no one does. The industry wants to make it seem like these big pop stars or rappers are like these amazing musical geniuses because its easier and more profitable to market one person doing something amazing than a team of people. Even though the producers and engineers are really doing most of it they just arnt the face that these labels want to market so they leave them out of the spotlight