But he likes to go back to it and play it some more, and I can't understand why, when there's nothing in it to do again. It's not even a sandbox, so you can't be silly in it, like you can in GTA or Red Dead.
He probably just likes the story or setting enough to play it over and over again.
It's the same way you can rewatch a movie, reread a book, listen to the same song twice, or take a walk in the same park again. Sure, you know what it's going to be and you've already experienced it before, but that doesn't make it boring.
As far as I know, no other video game has replicated a mid 20th century LA to that level of detail before. Maybe he just likes walking around the city and appreciating the authenticity of it's layout or the work the artists put into making a period-realistic city.