The fact that they are so difficult that only M's who enjoy torturing themselves enjoy them.
Jkjk in all seriousness it really comes down to the difficulty of the games and feeling like a badass when you kill things or the sense of achievement after you beat a boss.
I would add that the PVP can be very fun, especially in Bloodborne. Dark Souls Remastered PVP is good too. There are enough options that you can find something to suit your playstyle.