many reasons
1. they cant rap anything else cuz their lyrical abilities suck
2. its what sells and what everyone in mainstream wants to hear so to get money thats what you gotta do
but i dont care about money so i will not rap about that
and you dont have to make gun this gun that songs to show that your hard or real...your more real for showing your soft side everyones tryin to be all big and bad but only a coward would boast about not encountering fear <werd learned that from somewhere i forgot who said it