I’d say a few things
For Kendrick specifically, his lyrics are EXTREMELY personal in nature. He probably “feels” it more when he sings hooks and lyrics himself and can give you that level of emotion. I wouldn’t necessarily want to hear Jill Scott or D’angelo on a song like Mirrors because the emotion and feeling that he puts it into singing those words is a major part of what draws me to the song.
Probably cheaper and more cost effective to sing yourself then have to split publishing and pay feature fees when your fanbase isn’t complaining about you doing your own melodies
Also Kendrick is heavily influenced by Andre 3000, who literally made a career out of being one of the greatest rappers/decent singers of ALL time. He’s also influenced by Lil Wayne, who blew up using the same formula. Modern day artists like Drake, Childish Gambino, Big Krit, and Big Sean have done it to successful effect, so why not Kendrick?