Umm that's not what RPG means at all, that's an MMO, which this game is not. This is like Fallout or Skyrim, meant to be solo. RPG has definitely lost its meaning over the years to people.
How would that work? The game is not that big like a MMO, and has a very linear system. There are separated acts. You don't farm either, it's not Diablo. Enemies are perma-dead.
Your levels wouldn't match up and really mess up the game (you'd either be overpowered or underleveled and useless), or you'd have to find someone in the same place as you. and well, it just wouldn't work story and progression-wise. What happens when you join a game, get the xp, and go back to your solo save file that's still in Act 1 but now you're severely overleveled?
Imagine playing Fallout 4 or Skyrim, and joining a random person on their save. It makes no sense lol.