Some non-expert reasons

1. Crappy schedule--

ND doesn't pour it on after the game is in hand. We tend to tone it down. Problem is, there's never a big enough lead to get the 2nd/3rd team in to get reps. Offense stalls (on purpose). Shorter fields (because of a good defense) lowers total offensive yardage totals.

2. Predictable offense--

Bad teams can't overcome the athletes/talent ND has, so simple offense can score enough to win comfortably (see point #1). There's no urgency to make the offense more varied/unpredictable since it's good enough 10 out of 12 games. The losses (against great teams) are explained away with "we didn't make plays".

3. No QB "development"--

QBs never develop (I forget who made the point that the QBs don't regress. There's more tape on them as the season goes on, and into year 2 to the better teams can take away the few things the QB does well). Goes back to point #2. QBs don't grow because the offense is stagnant, and don't want to take more chances to make the throws they should.

4. Short WR bench--

Back to point #3. Few excellent receivers (or even very good) make it less likely the QBs will want to make the throws they should. No WR production means less attention to them and more to the rest of the offense--more bodies to take away TE routes, support in run game, etc.

You've known the problems for 10 years. They haven't changed.