Why do so many rap songs (especially modern ones)...

I reckon it's what's come to be expected of them. They're not serious rappers if they don't use those words and they won't make the cut on the 'street'. That, and a mix of lack of vocabulary and cliché-ridden laziness. 

It's the youth on the streets who always made me laugh. Going about with their jeans halfway down their ass and quoting these gangsta rhymes and looking mean, rapping about the feds, their cribs, their fancy rides, their 'holes', their gold chains - in fact reciting anything their heroes care to rap about, even the less glamorous stuff like racism, killings, ghettoes, poverty, welfare and all that. While the kids and youth go back to their ordinary slums and microwave dinners they forget that these wordy saviours are jollying it up at some Michelin starred restaurant or enjoying a private party in their own mansion which is tucked away in the hills with 24 hour patrols and 15 feet high walls, where all that 'street stuff' is forgotten - until the next tour and album.

That's my quick view.        

It's just posturing. They want to sound more "street" or "tough" as opposed to sounding like Will Smith.

Truth!

I remember in the late '90s and early '00s there was a push to make dance/techno/ambient whatever you want to call it the next thing, but it never really went mainstream here in the USA. That's the last real push for a "new thing" that I can remember. It's been more or less the same for a long time.