Yeah, I play guitar, drums and keyboards. I'm very bad at all those things but I know the basics. As a teenager I had a phase where I would write and rap and on my very biased opinion I was actually somewhat good at it.
I can't sing though.
I'm not going to adress the other stuff you're bringing up 'cause it's clear you're not getting the point I'm trying to make. What I'm saying is that is a lot easier to come up with decent sounding hip-hop tracks than decent sounding songs on other types of styles.
Unless you have no sense of rhythm at all is usually easier to rhyme words with a somewhat decent delivery than actually having the ability to sing.
You need a lot more musical knowledge to make a group of instruments sound good and armonic than coming-up with decent sounding beats on a machine.
I think most reasonable people would agree that is a lot more feasible for rock (or any other style of music) artists to fool around with keyboards, machines and lyrics and come up with decent raps than is for most rap artists to fool around with guitars, drums, basses, pianos, etc. and come up with decent songs where they actually have to sing.
If you don't think that then you either wrong or you are just arguing for the sake of arguing.