This is likely your personal perception and not indicated in any of the gameplay in any of the Diablo games.
All the gameplay of the Diablo games expresses the linear statistical progression of a character as both the cause and goal of gameplay. In practice the most efficient means of progression optimally leverages the specific repetition of narrow content; farming a particular zone, monster or activity.
What the older Diablo games lacked were multi modes which compartmentalized these activities expressed in gameplay.
What you are saying is incorrect.
No gameplay or design of the Diablo series indicates or expresses that repetitious playing of the campaign (which as a concept doesn't exist in D1 and D2 actually) or "main story" is a goal and/or cause of gameplay.
How you feel about gameplay is a matter of opinion. Gameplay itself is an object reality.