Actually, I understand that. But for most D3 player, game is not finished after campaign, and that is clearly what Blizzard have intended with the design of D3. Not saying it is ideal, but is the truth as I see it.
So by only playing the campaign, you don’t have a clue about what D3 is. I still say “go do GR 120 with 800 paragon”, if you sincerely want to experience challenge in D3.