You haven't read much poetry if you think rap is it.
Very few rap lyrics reach the level of poetry in my opinion. Poetry is about saying a lot with as few words as possible. By "a lot" I mean it has a deep meaning, not that it is literally condensing 100 words into 10.
Compare Future's "Fuck Up Some Commas":
Fuck up the llama, I'm hot like a sauna, yeah
The burner is on me, can't play with that money, yeah
I'm fucking with shorty, I'm loving the way that she fucking me yeah
Give no fucks yeah, we don't give no fucks, yeah
Throw my cup yeah, I just bought my plug, yeah
Full of the mud, yeah, nigga get full of them drugs, yeah
Feeling real right, yeah, I'm on same thing as Mike, yeah
Bought a lil ice, yeah, I wanna beat that dope like Ike, yeah
With... Lets say Naglfar's "The Brimstone Gate":
I have seen the most beautiful light since my birth
It's been possessing my mind
I've lashed myself
I have opened my veins, cursed my blood in grief...
As a shadow of death
The reaper from the tales of past
I've been lurking in the shades of thy creation
Feared by most
Hated by all, loved by none, a destiny of solitude
Very different themes, sure, but which is more poetic? The song that is quite literally about being too rich to care and buying/doing all the drugs/bitches, or the one that uses diabolical symbolism to describe a personal alienation?
I'm not saying other types of music are always poetic, just saying that while rap always is said to be very deep and poetic, I rarely think it rises to the occasion. Rap is most often literal, not poetic. Nothing wrong with that, of course.