They don't "perfect" theyr voices in rap. Rap isn't 50 cent, lil wayne or akon, those belongs to commercial music. Rap isnt "yo yo bitchez" eather, as said, usually commercial rap is like that. You obviously have bad taste in music, since you don't know the basics U.U
Is rock and heavy only screaming satanistic words? no. Is rapping about bitchez money and fame? no.
As well said, every genre got it good songs and it bad songs, and obviously, you donät know it.