Blizzard has explicitly stated that the game is designed from the ground up as an online-only experience. Several core design choices make an offline mode technically and commercially difficult:
By storing character data and game logic on Blizzard's servers rather than locally, the developers can better prevent hacking, item duplication, and gear modding. diablo iv offline mode
While you cannot play offline, you can play the vast majority of Diablo IV entirely solo. Blizzard Forums Offline D4 for Console but not PC? - Blizzard Forums Blizzard has explicitly stated that the game is