It's probably because rap music is basically word s just thrown together. And I'm not anti-rap by any means, though I'm not into it either. But they select their wording more based on the sounds of the words and how they go together and flow as opposed to what they actually mean. It's not at all uncommon for words to be conjugated incorrectly or used in an awkward context. So finding a meaning can be tricky there. Plus, from most of what I've heard over the years, especially newer stuff, there's little, if any meaning to be found. It's really all about personal conflicts with various other groups, acquiring material possessions, and flaunting said possessions.
As far as how meaning is found, I think there's so many different styles of lyrics that it's hard to pin down to one exact way. Some lyricists tend to be very cryptic and it's hard to find the meaning. It's almost written as a riddle for people to solve. At least that's how I've always seen it with those types.
Often, if a song actually has a chorus or any refrain, that's the best place to start when searching for the meaning. It's usually like a summary statement of sorts.
But most people in any form of even remotely popular music, at least that I've been exposed to, put very little effort into lyrics. Most of the time it's just something to scream or sing so the music has a voice in it. Most are actually very straightforward and express very unoriginal feelings, ideas or perspectives.
I know I've gotten to a point where I scarcely even bother finding out the lyrics. Occasionally a song or two comes around that is something really good in that way, but usually it's all the same stuff based on genre.
Metal= hatred for God or songs about rejection. Violence. Anger.
Rap= hatred for police, authority and love of possessions. Pride
Country= Good times, pickup trucks and loving the good ol' USA.
Pop= Lust, love and heartbreak. Frivolity.
When in doubt, that's most likely what a song means...