Superbikes are based on production motorcycles. Only allowed to change certain things.
MotoGP bikes are bespoke prototypes, all carbon, titanium and unobtanium. Nothing is allowed to be from a road bike, all is handmade to be the lightest/strongest possible.
MotoGP bikes are incredibly hard to ride, they're so much lighter, faster and better handling and braking than superbikes, most can't get the hang of it. Just because you dominate every other motorcycle racing class on earth doesn't mean you'll be very good at MotoGP. This has been proven many times.
I don't think road parts are not allowed per say, they just suck compared to what is allowed. Otherwise, spot on.