Rapping's been going on for longer than 50 years when DJ Cool Herc was making his mark. Before that, you had MCs like Count Machuki toasting, plus historically, chanting and speaking over beats has been a part of African Culture for millennia (eg, Griots from West Africa).
Actually performing or recording is as vocally challenging as singing - in some ways, it's much harder, as it requires a lot of vocal dexterity to maintain the flow for such a length of time. Even when it doesn't seem like it to the casual listener, there's great attention to pitch, tone, timbre, rhythm, etc, in those words. And they've got to write the lyrics. That's a fuckton of poetry that doesn't just have to work on a page, it has to work with the beats, the chords, everything. Oh, and for people of Stormzy's calibre, there's also composing the rest of the track, rather than just pasting in a couple of loops on Logic and spitting some bars.
Other than a party piece when 3 sheets to the wind of NWA's Fuck Da Police, whilst I'm a pretty good singer, there's no way I could maintain the physical, never mind mental, effort of rapping. Gilbert and Sullivan are difficult enough for most white people.
It's part of my job to understand music of all kinds. I'd be pretty shit at my job if I didn't. And Grime certainly counts as music - I might not be the target demographic, but that doesn't stop it being music just because a middleaged white woman says so.