A guess, and it’s only a guess, is that many of them might have gotten their love of Sinatra’s singing from their mamas. That’s how I got mine, and I’m about the same age as the first generation of rappers; he was her musical idol from her WWII bobby-soxer days until she died, a bit too young, in the mid 1990s.
I didn’t enjoy Frank Sinatra until late in life, after he was dead.
The guy could sing.