My opinion on this is that most people unwittingly believe they cant enjoy the game and experience all it has to offer without trading. This is almost definitely because of Diablo 2 and how long it took to figure out how to effectively farm. My opinion about this was reinforced when I, and many other people went back to D2R with the same assumptions based on lack of knowledge. The fact of the matter is in D2 specifically it's easier to get your ultra builds in offline single player. But most people dont know this and dont seek out the knowledge. ARPGs are traditionally some of the most nuanced games you could ever play, and the sweeping, majority opinion about almost all of them is you have to either trade or cheat to get anywhere.
And it's a bit foolish, as a game developer, to ignore your audience. Even if they are misguided. Plus, let's be honest... it's fun to hack and slash for loot with friends