POE will be online-only from now until it becomes abandonware.

It's pretty standard for games set up like this. I'm pretty sure because of this they also take a strict stance against modders and would order shutdown of private servers and take legal action against those or anyone who hacked the game to run offline. Offline play would let you mod and hack-in whatever you wanted with no money to them. The game may be free to play, but GGG is still a company and the only way they make money is if you buy things for POE.

If you can convince GGG that enough people want to play an offline character and would never ever ever be upset if they thought 'man this guy would be kick-ass in a group' and realized that was impossible, and had to play the entire game without ever trading with anyone or being able to access their online stash from their offline character, MAYBE the mode could be enabled. Online characters would need to be completely segregated from offline characters to prevent modding and cheating making their way into the online experience. The main reason this game doesn't have an offline mode is because the developers want you to be able to play any character either solo or multiplayer at any time.

Diablo 2 offline steam