I am quoting from a Reddit link,
Having to be connected to a server to play alone sucks. And thousands of people are complaining about that today... yet most of them don't understand why Blizzard doesn't offer online play. They say it's to prevent piracy, when it's not.
Blizzard has to run servers for at least 10 years to support this game, as they know from their past franchises, for the (very popular) multiplayer component. It shouldn't be surprising they looked for a way to get some recurring revenue from the game so that the ongoing costs are covered by the people still playing it. Their solution was the real-money auction house.
To run a real-money auction house, they have to absolutely eliminate hacking and duping, or rare items won't be rare and there will be no incentive to create a robust market which Blizzard can earn money from. The only sure way to eliminate hacking and duping is to run D3 like an MMO... put everything on a server you don't control. You can hack your client all day long, but since your characters, the world, mob spawns and deaths, experience and loot are all generated and stored on the server, you can't cheat and create items.
There is no single player game. They never programmed one. To offer one at this point would be as much design and programming effort as creating Diablo 4. So don't expect there to ever be an offline patch for D3. When you play single player, you're just playing a multiplayer server instance that happens to only have one player in it.
I fully support this.
( I found a thread in d3 forums where someone had linked this topic/post thought I would put it in a thread for all to see)