For close to two decades, groups with a message such as Public Enemy, NWO, Run DMC (who were more tongue in cheek), Kool Moe Dee, Salt-n-Pepa, LL Cool J, Snoop Dogg, Tupac in the 90s and so many others produced rhythmic and lyrical music. Some were political/social statements and some were just fun.
(and I'll agree that James Brown was the king).
Then came a generation of self-indulgent, "me generation" types and I can't explain how the music and the business changed except for the greed of the "artists" and their management, and the record labels.
The "formula" I mentioned earlier was created and the mass producing of music and music videos (everyone featuring big booties and the Hip-Hop stars living the good life while still being a thug and maintaining "street-cred") flooded the market. This formula includes a "one style fits all" type of Rap/Hip-Hop with heavy Auto Tune as a major part of the track. Only a handful of today's artists have anything to say; most songs are about being a thug, tapping that ass, bragging about sexual prowess and wealth.
And as mentioned earlier, it's mostly young White men and teens who are spending the money supporting this genre. Look at how many suburban HS kids dress the part and show no respect for women.
One note on Rap and Hip-Hop artists having no musical ability, I'd have to agree when talking about today's Rap. When this criticism arose back in the day, the Beastie Boys started to appear live playing their own instruments, which caused other Rap groups to tour with live instruments.